Communication system

ABSTRACT

Provided is a high-speed communication system with the high reliability and the low latency under New Radio (NR). A base station device includes a plurality of distributed units (DUs,  802 ) that transmit and receive radio signals, and a central unit (CU,  801 ) that controls the plurality of DUs ( 802 ). The CU ( 801 ) duplicates a downlink packet addressed to a communication terminal device ( 804 ), and forwards the duplicated downlink packet to each of at least two DUs ( 802 ) among the plurality of DUs ( 802 ). Each of the at least two of the DUs ( 802 ) transmits, to the communication terminal device ( 804 ) by the radio signal, the downlink packet obtained from the CU ( 801 ). Upon redundant receipt of the downlink packets, the communication terminal device ( 804 ) removes a redundant downlink packet in accordance with a predefined downlink packet removal criterion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 16/500,006, filed Oct. 1, 2019, which is a National StageApplication of International Application No. PCT/JP2018/016287, filedApr. 20, 2018, which is based upon and claims the benefit of priorityfrom Japanese Patent Application No. 2017-088666, filed on Apr. 27,2017, the entire contents of each of which are incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a communication system in which radiocommunication is performed between a communication terminal device suchas a user equipment device and a base station device.

BACKGROUND ART

The 3rd generation partnership project (3GPP), the standard organizationregarding the mobile communication system, is studying communicationsystems referred to as long term evolution (LTE) regarding radiosections and system architecture evolution (SAE) regarding the overallsystem configuration including a core network and a radio access networkwhich is hereinafter collectively referred to as a network as well (forexample, see Non-Patent Documents 1 to 5). This communication system isalso referred to as 3.9 generation (3.9 G) system.

As the access scheme of the LTE, orthogonal frequency divisionmultiplexing (OFDM) is used in a downlink direction and single carrierfrequency division multiple access (SC-FDMA) is used in an uplinkdirection. Further, differently from the wideband code division multipleaccess (W-CDMA), circuit switching is not provided but a packetcommunication system is only provided in the LTE.

The decisions taken in 3GPP regarding the frame configuration in the LTEsystem described in Non-Patent Document 1 (Chapter 5) are described withreference to FIG. 1. FIG. 1 is a diagram illustrating the configurationof a radio frame used in the LTE communication system. With reference toFIG. 1, one radio frame is 10 ms. The radio frame is divided into tenequally sized subframes. The subframe is divided into two equally sizedslots. The first and sixth subframes contain a downlink synchronizationsignal per radio frame. The synchronization signals are classified intoa primary synchronization signal (P-SS) and a secondary synchronizationsignal (S-SS).

Non-Patent Document 1 (Chapter 5) describes the decisions by 3GPPregarding the channel configuration in the LTE system. It is assumedthat the same channel configuration is used in a closed subscriber group(CSG) cell as that of a non-CSG cell.

A physical broadcast channel (PBCH) is a channel for downlinktransmission from a base station device (hereinafter may be simplyreferred to as a “base station”) to a communication terminal device(hereinafter may be simply referred to as a “communication terminal”)such as a user equipment device (hereinafter may be simply referred toas a “user equipment”). A BCH transport block is mapped to foursubframes within a 40 ms interval. There is no explicit signalingindicating 40 ms timing.

A physical control format indicator channel (PCFICH) is a channel fordownlink transmission from a base station to a communication terminal.The PCFICH notifies the number of orthogonal frequency divisionmultiplexing (OFDM) symbols used for PDCCHs from the base station to thecommunication terminal. The PCFICH is transmitted per subframe.

A physical downlink control channel (PDCCH) is a channel for downlinktransmission from a base station to a communication terminal. The PDCCHnotifies of the resource allocation information for downlink sharedchannel (DL-SCH) being one of the transport channels described below,resource allocation information for a paging channel (PCH) being one ofthe transport channels described below, and hybrid automatic repeatrequest (HARD) information related to DL-SCH. The PDCCH carries anuplink scheduling grant. The PDCCH carries acknowledgement(Ack)/negative acknowledgement (Nack) that is a response signal touplink transmission. The PDCCH is referred to as an L1/L2 control signalas well.

A physical downlink shared channel (PDSCH) is a channel for downlinktransmission from a base station to a communication terminal. A downlinkshared channel (DL-SCH) that is a transport channel and a PCH that is atransport channel are mapped to the PDSCH.

A physical multicast channel (PMCH) is a channel for downlinktransmission from a base station to a communication terminal. Amulticast channel (MCH) that is a transport channel is mapped to thePMCH.

A physical uplink control channel (PUCCH) is a channel for uplinktransmission from a communication terminal to a base station. The PUCCHcarries Ack/Nack that is a response signal to downlink transmission. ThePUCCH carries a channel quality indicator (CQI) report. The CQI isquality information indicating the quality of received data or channelquality. In addition, the PUCCH carries a scheduling request (SR).

A physical uplink shared channel (PUSCH) is a channel for uplinktransmission from a communication terminal to a base station. An uplinkshared channel (UL-SCH) that is one of the transport channels is mappedto the PUSCH.

A physical hybrid ARQ indicator channel (PHICH) is a channel fordownlink transmission from a base station to a communication terminal.The PHICH carries Ack/Nack that is a response signal to uplinktransmission. A physical random access channel (PRACH) is a channel foruplink transmission from the communication terminal to the base station.The PRACH carries a random access preamble.

A downlink reference signal (RS) is a known symbol in the LTEcommunication system. The following five types of downlink referencesignals are defined as: a cell-specific reference signal (CRS), an MBSFNreference signal, a data demodulation reference signal (DM-RS) being aUE-specific reference signal, a positioning reference signal (PRS), anda channel state information reference signal (CSI-RS). The physicallayer measurement objects of a communication terminal include referencesignal received powers (RSRPs).

The transport channels described in Non-Patent Document 1 (Chapter 5)are described. A broadcast channel (BCH) among the downlink transportchannels is broadcast to the entire coverage of a base station (cell).The BCH is mapped to the physical broadcast channel (PBCH).

Retransmission control according to a hybrid ARQ (HARD) is applied to adownlink shared channel (DL-SCH). The DL-SCH can be broadcast to theentire coverage of the base station (cell). The DL-SCH supports dynamicor semi-static resource allocation. The semi-static resource allocationis also referred to as persistent scheduling. The DL-SCH supportsdiscontinuous reception (DRX) of a communication terminal for enablingthe communication terminal to save power. The DL-SCH is mapped to thephysical downlink shared channel (PDSCH).

The paging channel (PCH) supports DRX of the communication terminal forenabling the communication terminal to save power. The PCH is requiredto be broadcast to the entire coverage of the base station (cell). ThePCH is mapped to physical resources such as the physical downlink sharedchannel (PDSCH) that can be used dynamically for traffic.

The multicast channel (MCH) is used for broadcasting the entire coverageof the base station (cell). The MCH supports SFN combining of multimediabroadcast multicast service (MBMS) services (MTCH and MCCH) inmulti-cell transmission. The MCH supports semi-static resourceallocation. The MCH is mapped to the PMCH.

Retransmission control according to a hybrid ARQ (HARQ) is applied to anuplink shared channel (UL-SCH) among the uplink transport channels. TheUL-SCH supports dynamic or semi-static resource allocation. The UL-SCHis mapped to the physical uplink shared channel (PUSCH).

A random access channel (RACH) is limited to control information. TheRACH involves a collision risk. The RACH is mapped to the physicalrandom access channel (PRACH).

The HARQ is described. The HARQ is the technique for improving thecommunication quality of a channel by combination of automatic repeatrequest (ARQ) and error correction (forward error correction). The HARQis advantageous in that error correction functions effectively byretransmission even for a channel whose communication quality changes.In particular, it is also possible to achieve further qualityimprovement in retransmission through combination of the receptionresults of the first transmission and the reception results of theretransmission.

An example of the retransmission method is described. If the receiverfails to successfully decode the received data, in other words, if acyclic redundancy check (CRC) error occurs (CRC=NG), the receivertransmits “Nack” to the transmitter. The transmitter that has received“Nack” retransmits the data. If the receiver successfully decodes thereceived data, in other words, if a CRC error does not occur (CRC=OK),the receiver transmits “AcK” to the transmitter. The transmitter thathas received “Ack” transmits the next data.

The logical channels described in Non-Patent Document 1 (Chapter 6) aredescribed. A broadcast control channel (BCCH) is a downlink channel forbroadcast system control information. The BCCH that is a logical channelis mapped to the broadcast channel (BCH) or downlink shared channel(DL-SCH) that is a transport channel.

A paging control channel (PCCH) is a downlink channel for transmittingpaging information and system information change notifications. The PCCHis used when the network does not know the cell location of acommunication terminal. The PCCH that is a logical channel is mapped tothe paging channel (PCH) that is a transport channel.

A common control channel (CCCH) is a channel for transmission controlinformation between communication terminals and a base station. The CCCHis used in a case where the communication terminals have no RRCconnection with the network. In the downlink direction, the CCCH ismapped to the downlink shared channel (DL-SCH) that is a transportchannel. In the uplink direction, the CCCH is mapped to the uplinkshared channel (UL-SCH) that is a transport channel.

A multicast control channel (MCCH) is a downlink channel forpoint-to-multipoint transmission. The MCCH is used for transmission ofMBMS control information for one or several MTCHs from a network to acommunication terminal. The MCCH is used only by a communicationterminal during reception of the MBMS. The MCCH is mapped to themulticast channel (MCH) that is a transport channel.

A dedicated control channel (DCCH) is a channel that transmits dedicatedcontrol information between a communication terminal and a network on apoint-to-point basis. The DCCH is used when the communication terminalhas an RRC connection. The DCCH is mapped to the uplink shared channel(UL-SCH) in uplink and mapped to the downlink shared channel (DL-SCH) indownlink.

A dedicated traffic channel (DTCH) is a point-to-point communicationchannel for transmission of user information to a dedicatedcommunication terminal. The DTCH exists in uplink as well as downlink.The DTCH is mapped to the uplink shared channel (UL-SCH) in uplink andmapped to the downlink shared channel (DL-SCH) in downlink.

A multicast traffic channel (MTCH) is a downlink channel for trafficdata transmission from a network to a communication terminal. The MTCHis a channel used only by a communication terminal during reception ofthe MBMS. The MTCH is mapped to the multicast channel (MCH).

CGI represents a cell global identifier. ECGI represents an E-UTRAN cellglobal identifier. A closed subscriber group (CSG) cell is introducedinto the LTE, and the long term evolution advanced (LTE-A) and universalmobile telecommunication system (UMTS) described below.

The closed subscriber group (CSG) cell is a cell in which subscriberswho are allowed to use are specified by an operator (hereinafter, alsoreferred to as a “cell for specific subscribers”). The specifiedsubscribers are allowed to access one or more cells of a public landmobile network (PLMN). One or more cells to which the specifiedsubscribers are allowed access are referred to as “CSG cell(s)”. Notethat access is limited in the PLMN.

The CSG cell is part of the PLMN that broadcasts a specific CSG identity(CSG ID) and broadcasts “TRUE” in a CSG indication. The authorizedmembers of the subscriber group who have registered in advance accessthe CSG cells using the CSG ID that is the access permissioninformation.

The CSG ID is broadcast by the CSG cell or cells. A plurality of CSG IDsexist in the LTE communication system. The CSG IDs are used bycommunication terminals (UEs) for making access from CSG-related memberseasier.

The locations of communication terminals are tracked based on an areacomposed of one or more cells. The locations are tracked for enablingtracking the locations of communication terminals and callingcommunication terminals, in other words, incoming calling tocommunication terminals even in an idle state. An area for trackinglocations of communication terminals is referred to as a tracking area.

In 3GPP, base stations referred to as Home-NodeB (Home-NB; HNB) andHome-eNodeB (Home-eNB; HeNB) are studied. HNB/HeNB is a base stationfor, for example, household, corporation, or commercial access servicein UTRAN/E-UTRAN. Non-Patent Document 2 discloses three different modesof the access to the HeNB and HNB. Specifically, an open access mode, aclosed access mode, and a hybrid access mode are disclosed.

Further, specifications of long term evolution advanced (LTE-A) arepursed as Release 10 in 3GPP (see Non-Patent Documents 3 and 4). TheLTE-A is based on the LTE radio communication system and is configuredby adding several new techniques to the system.

Carrier aggregation (CA) is studied for the LTE-A system in which two ormore component carriers (CCs) are aggregated to support widertransmission bandwidths up to 100 MHz. Non-Patent Document 1 describesthe CA.

In a case where CA is configured, a UE has a single RRC connection witha network (NW). In RRC connection, one serving cell provides NASmobility information and security input. This cell is referred to as aprimary cell (PCell). In downlink, a carrier corresponding to PCell is adownlink primary component carrier (DL PCC). In uplink, a carriercorresponding to PCell is an uplink primary component carrier (UL PCC).

A secondary cell (SCell) is configured to form a serving cell group witha PCell, in accordance with the UE capability. In downlink, a carriercorresponding to SCell is a downlink secondary component carrier (DLSCC). In uplink, a carrier corresponding to SCell is an uplink secondarycomponent carrier (UL SCC).

A serving cell group of one PCell and one or more SCells is configuredfor one UE.

The new techniques in the LTE-A include the technique of supportingwider bands (wider bandwidth extension) and the coordinated multiplepoint transmission and reception (CoMP) technique. The CoMP studied forLTE-A in 3GPP is described in Non-Patent Document 1.

Furthermore, the use of small eNBs (hereinafter also referred to as“small-scale base station devices”) configuring small cells is studiedin 3GPP to satisfy tremendous traffic in the future. In an exampletechnique under study, a large number of small eNBs is installed toconfigure a large number of small cells, which increases spectralefficiency and communication capacity. The specific techniques includedual connectivity (abbreviated as DC) with which a UE communicates withtwo eNBs through connection thereto. Non-Patent Document 1 describes theDC.

For eNBs that perform dual connectivity (DC), one may be referred to asa master eNB (abbreviated as MeNB), and the other may be referred to asa secondary eNB (abbreviated as SeNB).

The traffic flow of a mobile network is on the rise, and thecommunication rate is also increasing. It is expected that thecommunication rate is further increased when the operations of the LTEand the LTE-A are fully initiated.

For increasingly enhanced mobile communications, the fifth generation(hereinafter also referred to as “5G”) radio access system is studiedwhose service is aimed to be launched in 2020 and afterward. Forexample, in the Europe, an organization named METIS summarizes therequirements for 5G (see Non-Patent Document 5).

The requirements in the 5G radio access system show that a systemcapacity shall be 1000 times as high as, a data transmission rate shallbe 100 times as high as, a data latency shall be one tenth ( 1/10) aslow as, and simultaneously connected communication terminals 100 timesas many as those of the LTE system, to further reduce the powerconsumption and device cost.

To satisfy such requirements, the study of 5G standards is pursued asRelease 14 in 3GPP (see Non-Patent Documents 6 to 10). The techniques on5G radio sections are referred to as “New Radio Access Technology” (“NewRadio” is abbreviated as NR), and the several new techniques are beingstudied (see Non-Patent Documents 11 to 14). Examples of such studiesinclude packet duplication using the DC or multi-connectivity(abbreviated as MC), and split of a gNB into a central unit (CU) and adistributed unit (DU).

PRIOR-ART DOCUMENTS Non-Patent Documents

-   Non-Patent Document 1: 3GPP TS36.300 V14.0.0-   Non-Patent Document 2: 3GPP S1-083461-   Non-Patent Document 3: 3GPP TR 36.814 V9.0.0-   Non-Patent Document 4: 3GPP TR 36.912 V13.0.0-   Non-Patent Document 5: “Scenarios, requirements and KPIs for 5G    mobile and wireless system”, ICT-317669-METIS/D1.1-   Non-Patent Document 6: 3GPP TR 23.799 V1.1.0-   Non-Patent Document 7: 3GPP TR 38.801 V14.0.0-   Non-Patent Document 8: 3GPP TR 38.802 V1.0.0-   Non-Patent Document 9: 3GPP TR 38.804 V1.0.0-   Non-Patent Document 10: 3GPP TR 38.912 V0.0.2-   Non-Patent Document 11: 3GPP R2-1700672-   Non-Patent Document 12: 3GPP R2-1700172-   Non-Patent Document 13: 3GPP R2-1700982-   Non-Patent Document 14: 3GPP R2-1701472-   Non-Patent Document 15: 3GPP TS 36.423 v14.2.0-   Non-Patent Document 16: 3GPP TS 36.311 v14.2.1-   Non-Patent Document 17: CPRI Specification V7.0-   Non-Patent Document 18: 3GPP R2-1701461-   Non-Patent Document 19: 3GPP R2-1700177-   Non-Patent Document 20: 3GPP TS 36.323 v14.2.0-   Non-Patent Document 21: 3GPP TS 36.322 v14.0.0-   Non-Patent Document 22: 3GPP R3-170266-   Non-Patent Document 23: 3GPP TS36.425 V13.1.1

SUMMARY Problems to be Solved by the Invention

Under NR, splitting a gNB into two units, that is, a central unit (CU)and a distributed unit (DU) and enabling the CU to be connected to aplurality of DUs is proposed to increase the number of accommodated UEsper gNB. Moreover, application of the packet duplication with which eachgNB transmits and receives the same packet using the configuration ofthe DC or the MC is also proposed to provide communication thatsatisfies the high reliability and the low latency under NR.

Duplicating a packet using a plurality of DUs is proposed under NR.However, the configuration of the DC or the MC is not applicable betweenor among DUs as it is, so that the communication through the packetduplication using the plurality of DUs cannot be provided. Thus, thecommunication that satisfies the high reliability and the low latencycannot be provided.

Moreover, with application of a routing method using a plurality of DUs,particularly, with the concurrent use of the DC and the CU-DU splitunder NR, the MgNB does not know which DU of the SgNB data should beforwarded to. Since the MgNB cannot transmit data to the DUs beingserved by the SgNB, a UE and a base station have a problem of failing toestablish a communication using the DUs of the SgNB. Consequently, theDC and the CU-DU split are not concurrently available in the 5G, and theuse efficiency of radio resources substantially decreases.

In view of the problems, one of the objects of the present invention isto provide a high-speed communication system with the high reliabilityand the low latency under NR.

Means to Solve the Problems

The present invention provides, for example, a communication systemincluding a communication terminal device, and a base station deviceconfigured to perform radio communication with the communicationterminal device, wherein the base station device includes: a pluralityof distributed units (DUs) that transmit and receive radio signals; anda central unit (CU) that controls the plurality of DUs, the CUduplicates a downlink packet addressed to the communication terminaldevice, and forwards the duplicated downlink packet to each of at leasttwo DUs among the plurality of DUs, each of the at least two DUstransmits, to the communication terminal device by the radio signal, thedownlink packet obtained from the CU, and upon redundant receipt of thedownlink packets, the communication terminal device removes a redundantdownlink packet in accordance with a predefined downlink packet removalcriterion.

Effects of the Invention

The present invention can provide a high-speed communication system withthe high reliability and the low latency under NR.

The objects, features, aspects and advantages of the present inventionbecome more apparent from the following detailed description of thepresent invention when taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating the configuration of a radio frame foruse in an LTE communication system.

FIG. 2 is a block diagram showing the overall configuration of an LTEcommunication system 200 under discussion of 3GPP.

FIG. 3 is a block diagram showing the configuration of a user equipment202 shown in FIG. 2, which is a communication terminal according to thepresent invention.

FIG. 4 is a block diagram showing the configuration of a base station203 shown in FIG. 2, which is a base station according to the presentinvention.

FIG. 5 is a block diagram showing the configuration of an MME accordingto the present invention.

FIG. 6 is a flowchart showing an outline from a cell search to an idlestate operation performed by a communication terminal (UE) in the LTEcommunication system.

FIG. 7 shows the concept of a cell configuration when macro eNBs andsmall eNBs coexist.

FIG. 8 illustrates, in the downlink communication, a configuration ofthe CU that duplicates a packet in a PDCP layer and forwards theduplicated packets to a plurality of DUs, the DUs, and the UE thatdetects redundant packets according to the first embodiment.

FIG. 9 illustrates a configuration of a bearer that passes through allthe DUs being served by the CU according to the first embodiment.

FIG. 10 illustrates a sequence for starting transmission and receptionof duplicated packets in the communication between the CU and the UEaccording to the first embodiment.

FIG. 11 illustrates the sequence for starting transmission and receptionof duplicated packets in the communication between the CU and the UEaccording to the first embodiment.

FIG. 12 illustrates another sequence for starting transmission andreception of duplicated packets in the communication between the CU andthe UE according to the first embodiment.

FIG. 13 illustrates another sequence for starting transmission andreception of duplicated packets in the communication between the CU andthe UE according to the first embodiment.

FIG. 14 is a sequence diagram when the RRC connection reconfiguration isperformed after a response to an instruction for starting communicationaccording to the first embodiment.

FIG. 15 is the sequence diagram when the RRC connection reconfigurationis performed after a response to an instruction for startingcommunication according to the first embodiment.

FIG. 16 illustrates a sequence when the CU instructs each of DUs and theUE without waiting for responses from the DUs and the UE according tothe first embodiment.

FIG. 17 illustrates the sequence when the CU instructs each of DUs andthe UE without waiting for responses from the DUs and the UE accordingto the first embodiment.

FIG. 18 illustrates a sequence for stopping transmission and receptionof duplicated packets in the communication between the CU and the UEaccording to the first embodiment.

FIG. 19 is a sequence diagram illustrating switching between the use DUsand communication using a plurality of DUs in the C-Plane according tothe first embodiment.

FIG. 20 is the sequence diagram illustrating switching between the useDUs and communication using a plurality of DUs in the C-Plane accordingto the first embodiment.

FIG. 21 is a sequence diagram illustrating operations upon failure ofthe RRC connection reconfiguration according to the first embodiment.

FIG. 22 is the sequence diagram illustrating operations upon failure ofthe RRC connection reconfiguration according to the first embodiment.

FIG. 23 is a sequence diagram illustrating operations upon failure of aninstruction for starting communication according to the firstembodiment.

FIG. 24 is the sequence diagram illustrating operations upon failure ofan instruction for starting communication according to the firstembodiment.

FIG. 25 is a sequence diagram illustrating operations when a response tostarting communication is undelivered from the DU to the CU according tothe first embodiment.

FIG. 26 is the sequence diagram illustrating operations when a responseto starting communication is undelivered from the DU to the CU accordingto the first embodiment.

FIG. 27 illustrates a sequence for the mobility between the DUs usingpacket duplication according to the first embodiment.

FIG. 28 illustrates the sequence for the mobility between the DUs usingpacket duplication according to the first embodiment.

FIG. 29 illustrates a configuration for duplicating a packet in the PDCPlayer using a plurality of DUs in the downlink communication, in Option3-1 of the CU-DU split according to the first modification of the firstembodiment.

FIG. 30 illustrates a configuration for duplicating a packet in an RLC-Hlayer using a plurality of DUs in the downlink communication, in Option3-1 of the CU-DU split according to the second embodiment.

FIG. 31 illustrates a configuration for PDCP acknowledgement usingHARQ-ACK according to the third embodiment.

FIG. 32 illustrates a sequence on the mobility for the PDCPacknowledgement using HARQ-ACK according to the third embodiment.

FIG. 33 illustrates the sequence on the mobility for the PDCPacknowledgement using HARQ-ACK according to the third embodiment.

FIG. 34 illustrates the sequence on the mobility for the PDCPacknowledgement using HARQ-ACK according to the third embodiment.

FIG. 35 illustrates another sequence on the mobility for the PDCPacknowledgement using HARQ-ACK according to the third embodiment.

FIG. 36 illustrates another sequence on the mobility for the PDCPacknowledgement using HARQ-ACK according to the third embodiment.

FIG. 37 illustrates another sequence on the mobility for the PDCPacknowledgement using HARQ-ACK according to the third embodiment.

FIG. 38 illustrates a configuration for RLC acknowledgement usingHARQ-ACK, in Option 3-1 of the CU-DU split according to the firstmodification of the third embodiment.

FIG. 39 is a sequence diagram illustrating the mobility betweensecondary base stations using a PDCP status report from the UE accordingto the fourth embodiment.

FIG. 40 is a sequence diagram illustrating the mobility between DUsusing a PDCP status report from the UE according to the fourthembodiment.

FIG. 41 is the sequence diagram illustrating the mobility between DUsusing a PDCP status report from the UE according to the fourthembodiment.

FIG. 42 illustrates an example architecture when the PDCP of the SgNB isprovided with the routing functions between DUs according to the sixthembodiment.

FIG. 43 illustrates an example architecture when the routing functionsbetween DUs are provided outside the PDCP in the CU of the SgNBaccording to the sixth embodiment.

FIG. 44 illustrates an example architecture when a protocol stack havingthe routing functions between DUs are provided outside the PDCP in theCU of the SgNB according to the sixth embodiment.

FIG. 45 illustrates an example sequence on the DC with the SB using theSgNB with the CU-DU split configuration according to the sixthembodiment.

FIG. 46 illustrates the example sequence on the DC with the SB using theSgNB with the CU-DU split configuration according to the sixthembodiment.

FIG. 47 illustrates the example sequence on the DC with the SB using theSgNB with the CU-DU split configuration according to the sixthembodiment.

FIG. 48 illustrates an example architecture when the MgNB is providedwith a function of determining a routing destination DU of the SgNB andthe PDCP in the CU of the SgNB is provided with the routing functionsbetween DUs according to the first modification of the sixth embodiment.

FIG. 49 illustrates an example architecture when one buffer for routingis provided in the CU of the SgNB according to the first modification ofthe sixth embodiment.

FIG. 50 illustrates an example architecture when the buffer for routingis provided in the CU of the SgNB for each DU according to the firstmodification of the sixth embodiment.

FIG. 51 illustrates an example sequence on the DC with the SB using theSgNB with the CU-DU split configuration according to the firstmodification of the sixth embodiment.

FIG. 52 illustrates the example sequence on the DC with the SB using theSgNB with the CU-DU split configuration according to the firstmodification of the sixth embodiment.

FIG. 53 illustrates the example sequence on the DC with the SB using theSgNB with the CU-DU split configuration according to the firstmodification of the sixth embodiment.

FIG. 54 illustrates an example architecture when the MgNB is providedwith the routing functions between DUs of the SgNB according to thesecond modification of the sixth embodiment.

FIG. 55 illustrates an example sequence on the DC with the SB using theSgNB with the CU-DU split configuration according to the secondmodification of the sixth embodiment.

FIG. 56 illustrates the example sequence on the DC with the SB using theSgNB with the CU-DU split configuration according to the secondmodification of the sixth embodiment.

FIG. 57 illustrates the example sequence on the DC with the SB using theSgNB with the CU-DU split configuration according to the secondmodification of the sixth embodiment.

DESCRIPTION OF EMBODIMENTS First Embodiment

FIG. 2 is a block diagram showing an overall configuration of an LTEcommunication system 200 which is under discussion of 3GPP. FIG. 2 isdescribed here. A radio access network is referred to as an evolveduniversal terrestrial radio access network (E-UTRAN) 201. A userequipment device (hereinafter, referred to as a “user equipment (UE)”)202 that is a communication terminal device is capable of radiocommunication with a base station device (hereinafter, referred to as a“base station (E-UTRAN Node B: eNB)”) 203 and transmits and receivessignals through radio communication.

Here, the “communication terminal device” covers not only a userequipment device such as a movable mobile phone terminal device, butalso an unmovable device such as a sensor. In the following description,the “communication terminal device” may be simply referred to as a“communication terminal”.

The E-UTRAN is composed of one or a plurality of base stations 203,provided that a control protocol for the user equipment 202 such as aradio resource control (RRC), and user planes (hereinafter also referredto as “U-planes”) such as a packet data convergence protocol (PDCP),radio link control (RLC), medium access control (MAC), or physical layer(PHY) are terminated in the base station 203.

The control protocol radio resource control (RRC) between the userequipment 202 and the base station 203 performs broadcast, paging, RRCconnection management, and the like. The states of the base station 203and the user equipment 202 in RRC are classified into RRC_IDLE andRRC_CONNECTED.

In RRC_IDLE, public land mobile network (PLMN) selection, systeminformation (SI) broadcast, paging, cell re-selection, mobility, and thelike are performed.

In RRC_CONNECTED, the user equipment has RRC connection and is capableof transmitting and receiving data to and from a network. InRRC_CONNECTED, for example, handover (HO) and measurement of a neighborcell are performed.

The base stations 203 are classified into eNBs 207 and Home-eNBs 206.The communication system 200 includes an eNB group 203-1 including aplurality of eNBs 207 and a Home-eNB group 203-2 including a pluralityof Home-eNBs 206. A system, composed of an evolved packet core (EPC)being a core network and an E-UTRAN 201 being a radio access network, isreferred to as an evolved packet system (EPS). The EPC being a corenetwork and the E-UTRAN 201 being a radio access network may becollectively referred to as a “network”.

The eNB 207 is connected to an MME/S-GW unit (hereinafter, also referredto as an “MME unit”) 204 including a mobility management entity (MME), aserving gateway (S-GW), or an MME and an S-GW by means of an S1interface, and control information is communicated between the eNB 207and the MME unit 204. A plurality of MME units 204 may be connected toone eNB 207. The eNBs 207 are connected to each other by means of an X2interface, and control information is communicated between the eNBs 207.

The Home-eNB 206 is connected to the MME unit 204 by means of an S1interface, and control information is communicated between the Home-eNB206 and the MME unit 204. A plurality of Home-eNBs 206 are connected toone MME unit 204. Or, the Home-eNBs 206 are connected to the MME units204 through a Home-eNB gateway (HeNBGW) 205. The Home-eNB 206 isconnected to the HeNBGW 205 by means of an S1 interface, and the HeNBGW205 is connected to the MME unit 204 by means of an S1 interface.

One or a plurality of Home-eNBs 206 are connected to one HeNBGW 205, andinformation is communicated therebetween through an S1 interface. TheHeNBGW 205 is connected to one or a plurality of MME units 204, andinformation is communicated therebetween through an S1 interface.

The MME units 204 and HeNBGW 205 are entities of higher layer,specifically, higher nodes, and control the connections between the userequipment (UE) 202 and the eNB 207 and the Home-eNB 206 being basestations. The MME units 204 configure an EPC being a core network. Thebase station 203 and the HeNBGW 205 configure the E-UTRAN 201.

Further, the configuration below is studied in 3GPP. The X2 interfacebetween the Home-eNBs 206 is supported. In other words, the Home-eNBs206 are connected to each other by means of an X2 interface, and controlinformation is communicated between the Home-eNBs 206. The HeNBGW 205appears to the MME unit 204 as the Home-eNB 206. The HeNBGW 205 appearsto the Home-eNB 206 as the MME unit 204.

The interfaces between the Home-eNBs 206 and the MME units 204 are thesame, which are the S1 interfaces, in both cases where the Home-eNB 206is connected to the MME unit 204 through the HeNBGW 205 and it isdirectly connected to the MME unit 204.

The base station device 203 may configure a single cell or a pluralityof cells. Each cell has a range predetermined as a coverage in which thecell can communicate with the user equipment 202 and performs radiocommunication with the user equipment 202 within the coverage. In a casewhere one base station device 203 configures a plurality of cells, everycell is configured so as to communicate with the user equipment 202.

FIG. 3 is a block diagram showing the configuration of the userequipment 202 of FIG. 2 that is a communication terminal according tothe present invention. The transmission process of the user equipment202 shown in FIG. 3 is described. First, a transmission data buffer unit303 stores the control data from a protocol processing unit 301 and theuser data from an application unit 302. The data stored in thetransmission data buffer unit 303 is passed to an encoding unit 304, andis subject to an encoding process such as error correction. There mayexist the data output from the transmission data buffer unit 303directly to a modulating unit 305 without the encoding process. The dataencoded by the encoding unit 304 is modulated by the modulating unit305. The modulated data is converted into a baseband signal, and thebaseband signal is output to a frequency converting unit 306 and is thenconverted into a radio transmission frequency. After that, atransmission signal is transmitted from an antenna 307 to the basestation 203.

The user equipment 202 executes the reception process as follows. Theradio signal from the base station 203 is received through the antenna307. The received signal is converted from a radio reception frequencyinto a baseband signal by the frequency converting unit 306 and is thendemodulated by a demodulating unit 308. The demodulated data is passedto a decoding unit 309, and is subject to a decoding process such aserror correction. Among the pieces of decoded data, the control data ispassed to the protocol processing unit 301, and the user data is passedto the application unit 302. A series of processes by the user equipment202 is controlled by a control unit 310. This means that, though notshown in FIG. 3, the control unit 310 is connected to the individualunits 301 to 309.

FIG. 4 is a block diagram showing the configuration of the base station203 of FIG. 2 that is a base station according to the present invention.The transmission process of the base station 203 shown in FIG. 4 isdescribed. An EPC communication unit 401 performs data transmission andreception between the base station 203 and the EPC (such as the MME unit204), HeNBGW 205, and the like. A communication with another basestation unit 402 performs data transmission and reception to and fromanother base station. The EPC communication unit 401 and thecommunication with another base station unit 402 each transmit andreceive information to and from a protocol processing unit 403. Thecontrol data from the protocol processing unit 403, and the user dataand the control data from the EPC communication unit 401 and thecommunication with another base station unit 402 are stored in atransmission data buffer unit 404.

The data stored in the transmission data buffer unit 404 is passed to anencoding unit 405, and then an encoding process such as error correctionis performed for the data. There may exist the data output from thetransmission data buffer unit 404 directly to a modulating unit 406without the encoding process. The encoded data is modulated by themodulating unit 406. The modulated data is converted into a basebandsignal, and the baseband signal is output to a frequency converting unit407 and is then converted into a radio transmission frequency. Afterthat, a transmission signal is transmitted from an antenna 408 to one ora plurality of user equipments 202.

The reception process of the base station 203 is executed as follows. Aradio signal from one or a plurality of user equipments 202 is receivedthrough the antenna 408. The received signal is converted from a radioreception frequency into a baseband signal by the frequency convertingunit 407, and is then demodulated by a demodulating unit 409. Thedemodulated data is passed to a decoding unit 410 and then subject to adecoding process such as error correction. Among the pieces of decodeddata, the control data is passed to the protocol processing unit 403,the EPC communication unit 401, or the communication with another basestation unit 402, and the user data is passed to the EPC communicationunit 401 and the communication with another base station unit 402. Aseries of processes by the base station 203 is controlled by a controlunit 411. This means that, though not shown in FIG. 4, the control unit411 is connected to the individual units 401 to 410.

FIG. 5 is a block diagram showing the configuration of the MME accordingto the present invention. FIG. 5 shows the configuration of an MME 204 aincluded in the MME unit 204 shown in FIG. 2 described above. A PDN GWcommunication unit 501 performs data transmission and reception betweenthe MME 204 a and the PDN GW. A base station communication unit 502performs data transmission and reception between the MME 204 a and thebase station 203 by means of the S1 interface. In a case where the datareceived from the PDN GW is user data, the user data is passed from thePDN GW communication unit 501 to the base station communication unit 502via a user plane communication unit 503 and is then transmitted to oneor a plurality of base stations 203. In a case where the data receivedfrom the base station 203 is user data, the user data is passed from thebase station communication unit 502 to the PDN GW communication unit 501via the user plane communication unit 503 and is then transmitted to thePDN GW.

In a case where the data received from the PDN GW is control data, thecontrol data is passed from the PDN GW communication unit 501 to acontrol plane control unit 505. In a case where the data received fromthe base station 203 is control data, the control data is passed fromthe base station communication unit 502 to the control plane controlunit 505.

A HeNBGW communication unit 504 is provided in a case where the HeNBGW205 is provided, which performs data transmission and reception betweenthe MME 204 a and the HeNBGW 205 by means of the interface (IF)according to an information type. The control data received from theHeNBGW communication unit 504 is passed from the HeNBGW communicationunit 504 to the control plane control unit 505. The processing resultsof the control plane control unit 505 are transmitted to the PDN GW viathe PDN GW communication unit 501. The processing results of the controlplane control unit 505 are transmitted to one or a plurality of basestations 203 by means of the S1 interface via the base stationcommunication unit 502, and are transmitted to one or a plurality ofHeNBGWs 205 via the HeNBGW communication unit 504.

The control plane control unit 505 includes a NAS security unit 505-1,an SAE bearer control unit 505-2, and an idle state mobility managingunit 505-3, and performs an overall process for the control plane(hereinafter also referred to as a “C-plane”). The NAS security unit505-1 provides, for example, security of a non-access stratum (NAS)message. The SAE bearer control unit 505-2 manages, for example, asystem architecture evolution (SAE) bearer. The idle state mobilitymanaging unit 505-3 performs, for example, mobility management of anidle state (LTE-IDLE state which is merely referred to as idle as well),generation and control of a paging signal in the idle state, addition,deletion, update, and search of a tracking area of one or a plurality ofuser equipments 202 being served thereby, and tracking area listmanagement.

The MME 204 a distributes a paging signal to one or a plurality of basestations 203. In addition, the MME 204 a performs mobility control of anidle state. When the user equipment is in the idle state and an activestate, the MME 204 a manages a list of tracking areas. The MME 204 abegins a paging protocol by transmitting a paging message to the cellbelonging to a tracking area in which the UE is registered. The idlestate mobility managing unit 505-3 may manage the CSG of the Home-eNBs206 to be connected to the MME 204 a, CSG IDs, and a whitelist.

An example of a cell search method in a mobile communication system isdescribed next. FIG. 6 is a flowchart showing an outline from a cellsearch to an idle state operation performed by a communication terminal(UE) in the LTE communication system. When starting a cell search, inStep ST601, the communication terminal synchronizes slot timing andframe timing by a primary synchronization signal (P-SS) and a secondarysynchronization signal (S-SS) transmitted from a neighbor base station.

The P-SS and S-SS are collectively referred to as a synchronizationsignal (SS). Synchronization codes, which correspond one-to-one to PCIsassigned per cell, are assigned to the synchronization signals (SSs).The number of PCIs is currently studied in 504 ways. The 504 ways ofPCIs are used for synchronization, and the PCIs of the synchronizedcells are detected (specified).

In Step ST602, next, the user equipment detects a cell-specificreference signal (CRS) being a reference signal (RS) transmitted fromthe base station per cell and measures the reference signal receivedpower (RSRP). The codes corresponding one-to-one to the PCIs are usedfor the reference signal RS. Separation from another cell is enabled bycorrelation using the code. The code for RS of the cell is derived fromthe PCI specified in Step ST601, so that the RS can be detected and theRS received power can be measured.

In Step ST603, next, the user equipment selects the cell having the bestRS received quality, for example, the cell having the highest RSreceived power, that is, the best cell, from one or more cells that havebeen detected up to Step ST602.

In Step ST604, next, the user equipment receives the PBCH of the bestcell and obtains the BCCH that is the broadcast information. A masterinformation block (MIB) containing the cell configuration information ismapped to the BCCH over the PBCH. Accordingly, the MIB is obtained byobtaining the BCCH through reception of the PBCH. Examples of the MIBinformation include the downlink (DL) system bandwidth (also referred toas a transmission bandwidth configuration (dl-bandwidth)), the number oftransmission antennas, and a system frame number (SFN).

In Step ST605, next, the user equipment receives the DL-SCH of the cellbased on the cell configuration information of the MIB, to therebyobtain a system information block (SIB) 1 of the broadcast informationBCCH. The SIB1 contains the information about the access to the cell,information about cell selection, and scheduling information on anotherSIB (SIBk; k is an integer equal to or greater than two). In addition,the SIB1 contains a tracking area code (TAC).

In Step ST606, next, the communication terminal compares the TAC of theSIB1 received in Step ST605 with the TAC portion of a tracking areaidentity (TAI) in the tracking area list that has already been possessedby the communication terminal. The tracking area list is also referredto as a TAI list. TAI is the identification information for identifyingtracking areas and is composed of a mobile country code (MCC), a mobilenetwork code (MNC), and a tracking area code (TAC). MCC is a countrycode. MNC is a network code. TAC is the code number of a tracking area.

If the result of the comparison of Step ST606 shows that the TACreceived in Step ST605 is identical to the TAC included in the trackingarea list, the user equipment enters an idle state operation in thecell. If the comparison shows that the TAC received in Step ST605 is notincluded in the tracking area list, the communication terminal requiresa core network (EPC) including MME and the like to change a trackingarea through the cell for performing tracking area update (TAU).

The device configuring a core network (hereinafter, also referred to asa “core-network-side device”) updates the tracking area list based on anidentification number (such as UE-ID) of a communication terminaltransmitted from the communication terminal together with a TAU requestsignal. The core-network-side device transmits the updated tracking arealist to the communication terminal. The communication terminal rewrites(updates) the TAC list of the communication terminal based on thereceived tracking area list. After that, the communication terminalenters the idle state operation in the cell.

Widespread use of smartphones and tablet terminal devices explosivelyincreases traffic in cellular radio communications, causing a fear ofinsufficient radio resources all over the world. To increase spectralefficiency, thus, it is studied to downsize cells for further spatialseparation.

In the conventional configuration of cells, the cell configured by aneNB has a relatively-wide-range coverage. Conventionally, cells areconfigured such that relatively-wide-range coverages of a plurality ofcells configured by a plurality of macro eNBs cover a certain area.

When cells are downsized, the cell configured by an eNB has anarrow-range coverage compared with the coverage of a cell configured bya conventional eNB. Thus, in order to cover a certain area as in theconventional case, a larger number of downsized eNBs than theconventional eNBs are required.

In the description below, a “macro cell” refers to a cell having arelatively wide coverage, such as a cell configured by a conventionaleNB, and a “macro eNB” refers to an eNB configuring a macro cell. A“small cell” refers to a cell having a relatively narrow coverage, suchas a downsized cell, and a “small eNB” refers to an eNB configuring asmall cell.

The macro eNB may be, for example, a “wide area base station” describedin Non-Patent Document 7.

The small eNB may be, for example, a low power node, local area node, orhotspot. Alternatively, the small eNB may be a pico eNB configuring apico cell, a femto eNB configuring a femto cell, HeNB, remote radio head(RRH), remote radio unit (RRU), remote radio equipment (RRE), or relaynode (RN). Still alternatively, the small eNB may be a “local area basestation” or “home base station” described in Non-Patent Document 7.

FIG. 7 shows the concept of the cell configuration in which macro eNBsand small eNBs coexist. The macro cell configured by a macro eNB has arelatively-wide-range coverage 701. A small cell configured by a smalleNB has a coverage 702 whose range is narrower than that of the coverage701 of a macro eNB (macro cell).

When a plurality of eNBs coexist, the coverage of the cell configured byan eNB may be included in the coverage of the cell configured by anothereNB. In the cell configuration shown in FIG. 7, as indicated by areference “704” or “705”, the coverage 702 of the small cell configuredby a small eNB may be included in the coverage 701 of the macro cellconfigured by a macro eNB.

As indicated by the reference “705”, the coverages 702 of a pluralityof, for example, two small cells may be included in the coverage 701 ofone macro cell. A user equipment (UE) 703 is included in, for example,the coverage 702 of the small cell and performs communication via thesmall cell.

In the cell configuration shown in FIG. 7, as indicated by a reference“706”, the coverage 701 of the macro cell configured by a macro eNB mayoverlap the coverages 702 of the small cells configured by small eNBs ina complicated manner.

As indicated by a reference “707”, the coverage 701 of the macro cellconfigured by a macro eNB need not overlap the coverages 702 of thesmall cells configured by small eNBs.

Further, as indicated by a reference “708”, the coverages 702 of a largenumber of small cells configured by a large number of small eNBs may beconfigured in the coverage 701 of one macro cell configured by one macroeNB.

One of the services in NR is Ultra Reliability, Low LatencyCommunication (URLLC) requiring the communication with the low latencyand the high reliability. In the 3GPP standardization meeting, noapplication of Acknowledged Mode (AM) in the RLC layer for the URLLC hasbeen agreed for satisfying both the low latency and the highreliability. Another agreement reached in the 3GPP standardizationmeeting is to support the packet duplication in the PDCP layer forensuring the reliability using Unacknowledged Mode (UM) in the RLC layer(see Non-Patent Document 11 (3GPP R2-1700672)). A proposal is made onapplying the packet duplication to the configurations of the DC and theMC in NR (see Non-Patent Document 12 (3GPP R2-1700172)).

Moreover, splitting a gNB into two units is proposed in 3GPP (seeNon-Patent Document 7). The two units are referred to as the centralunit (CU) and the distributed unit (DU). A plurality of DUs areconnected to the CU. Several options on sharing functions between the CUand the DU in the CU-DU split are proposed. For example, Option 2 isproposed in which the CU has the PDCP and the DU has the RLC, the MAC,and the PHY. Moreover, Option 3 is proposed in which the CU has the PDCPand the H-RLC and the DU has the L-RLC, the MAC, and the PHY. Option 3includes Option 3-1. Option 3-1 is proposed in which the L-RLC in Option3 has a function of RLC-PDU split and the H-RLC in Option 3 has anacknowledgement function and the other functions of the RLC.

A proposal is made on applying the DC or the MC to communication using aplurality of DUs in NR (see Non-Patent Document 13 (3GPP R2-1700982) andNon-Patent Document 14 (3GPP R2-1701472)).

Since the DU has no distinction between a master base station and asecondary base station in the DC and the MC, the configuration and thesequence for the DC or the MC are not applicable to the communicationbetween the DUs as they are. Thus, a problem of failing to establishcommunication using a plurality of DUs, and a problem of a communicationfailure between a base station and a UE occur.

Since a mobility sequence between the DUs is not disclosed, the UEcannot switch the corresponding DU to another even when the UE moves.Thus, upon movement of the UE, a problem of failing to provide a stablecommunication occurs.

Moreover, since the configuration and the sequence for the DC or the MCare not applicable to the communication between DUs as they are, the CUcannot provide communication through the packet duplication using aplurality of DUs. Thus, a problem of failing to provide thecommunication that satisfies the high reliability and the low latencyoccurs.

The first embodiment discloses a method for solving such problems.

The CU duplicates a packet forwarded from a high-level network device.The CU forwards the duplicated packet to each DU. Each of the DUstransmits the packet to the UE. The UE detects redundant packets. The UEremoves the redundant packets. The UE may, for example, validate onlyone of the identical redundant packets received and remove the rest ofthe packets in the detection and the removal of redundant packets.

The CU may duplicate the packet in the PDCP layer. The UE may detect andremove the redundant packets in the PDCP layer.

The operations of the CU, the DUs, and the UE may be performed in thedownlink communication.

The UE duplicates a packet, and transmits the duplicated packet to eachof the DUs through a lower-layer entity that corresponds to the DU. Eachof the DUs forwards the received packet to the CU. The CU detectsredundant packets received from the DUs. The CU removes the redundantpackets. The CU may, for example, validate only one of the identicalredundant packets received and remove the rest of the packets in thedetection and the removal of redundant packets. The CU forwards thepacket that is not removed to the high-level network device.

The operations of the CU, the DUs, and the UE may be performed in theuplink communication.

The CU may forward the duplicated packets to all the DUs being servedthereby. Similarly, the UE may forward the duplicated packets to all theDUs being served by the corresponding CU. This can increase theredundancy and the reliability of the communication.

Alternatively, the CU may forward the duplicated packets to a part ofthe DUs being served thereby. Similarly, the UE may transmit theduplicated packets to a part of the DUs being served by the CU. This canefficiently enhance the reliability of the communication.

A DU that forwards the duplicated packet through the CU (hereinafter maybe referred to as a “use DU in the downlink communication”) may bedifferent from a DU that transmits the duplicated packet through the UE(hereinafter may be referred to as a “use DU in the uplinkcommunication”). The number of use DUs in the downlink communication maybe different from the number of use DUs in the uplink communication.Consequently, a communication path can be flexibly set.

The number of use DUs in the downlink communication may be one, two, ormore than or equal to three. The number of use DUs in the uplinkcommunication may be the same as that in the downlink communication.This can efficiently enhance the reliability.

The use DUs in the downlink communication and the use DUs in the uplinkcommunication (hereinafter may be referred to as “use DUs”) may be setto each CU. For example in communication with a UE, the CU #1 may applythe DUs #1 and #2 among the DUs #1 to #3 being served thereby, and theCU #2 may apply the DUs #5 and #6 among the DUs #4 to #6 being servedthereby. Consequently, the optimal communication system can be builtaccording to the communication paths between each CU and the DUs.

The use DUs may be set to each UE. For example, when a CU has the DUs #1to #3 being served thereby, the CU may apply the DUs #1 and #2 in thecommunication with the UE #1. The CU may apply the DUs #2 and #3 in thecommunication with the UE #2. Consequently, the optimal communicationsystem can be built according to the position of the UE.

The use DUs may be set according to each combination of CUs and UEs. Forexample in the communication with the UE #1, the CU #1 may apply the DUs#1 and #2 among the DUs #1 to #3 being served thereby, and the CU #2 mayapply the DUs #4 and #6 among the DUs #4 to #6 being served thereby. Inthe communication with the UE #2, the CU #1 may apply the DUs #2 and #3,and the CU #2 may apply the DUs #4 to #6. Consequently, the optimalcommunication system can be built according to the position relationshipand the communication paths between the CUs and the DUs.

The CU may determine the use DUs.

Examples of (1) to (5) below are disclosed as information to be usedwhen the CU makes the determination:

(1) measurement results obtained by the DUs being served thereby, forexample, measurement results of uplink signals;

(2) a measurement result obtained by the UE, for example, a measurementresult of a downlink signal;

(3) load states of the DUs being served thereby;

(4) a load state of the CU; and

(5) combinations of (1) and (4) above.

Since (1) eliminates the need for the UE to notify the CU of themeasurement results, the amount of signaling in the radio section can bereduced.

The DUs may measure the uplink reference signal (uplink RS) in (1).Consequently, the DUs can measure the uplink signal irrespective of thepresence or absence of the user data. Alternatively, the DUs may measurean error rate of the uplink data. Consequently, the DUs can reduce theprocessing time for measuring the uplink signal. The DUs may measure, asthe error rate, a bit error rate (BER) before an error correctionprocedure. Consequently, the DUs can obtain the measurement results thatproperly reflect radio channel states. The DUs may measure a block errorrate (BLER) after the error correction procedure. Consequently, the DUscan shorten the processing time for obtaining the measurement results.

Since (2) enables the use of the same process as that for obtaining themeasurement result in the existing LTE communication system, thecomplexity in designing a communication system can be avoided.

The UE may measure the downlink reference signal (downlink RS) in (2).Consequently, the UE can obtain a measurement result of the downlinksignal irrespective of the presence or absence of the user data.Alternatively, the UE may measure an error rate of the downlink data.Consequently, the UE can reduce the processing time for measuring theuplink signal. The UE may measure, as the error rate, a bit error rate(BER) before an error correction process. Consequently, the UE canobtain a measurement result that properly reflects a radio channelstate. The UE may measure a block error rate (BLER) after the errorcorrection procedure. Consequently, the UE can shorten the processingtime for obtaining the measurement result.

The UE may notify the CU of information of (2). The UE may notify theinformation through the DUs. The UE may notify the DUs of theinformation via the L1/L2 signaling. This enables a prompt notificationaccording to change in a channel state. Alternatively, the MAC signalingmay be used for the notification. Since the multi-level modulations areapplicable to the MAC signaling, the number of symbols can be reduced.Alternatively, the RRC signaling may be used for the notification. Theuse of the RRC signaling facilitates the process of forwarding theinformation from the DUs to the CU.

When the UE notifies the information to the CU, the DU on which the UEhas been camping may be the use DU. Consequently, it is possible toshorten the time required for the UE to be connected to the DU.

Alternatively, the DU whose connection with the UE has been establishedmay be the use DU. The DU whose connection with the UE has beenestablished may be, for example, a DU that has completed a random accessprocedure. Alternatively, for example, a DU with which the RRCconnection has been established may be used. Consequently, for example,even if the DU on which the UE has been camping is different from the DUwith which the connection has been currently established due to theoccurrence of mobility, the UE can give the notification to the CU.

Alternatively, when the UE notifies the information to the CU, aplurality of DUs may be the use DUs. The plurality of DUs may be, forexample, a combination of the DU on which the UE has been camping andthe DU whose connection with the UE has been established, or a pluralityof DUs whose connection with the UE has been established. This enhancesthe reliability of the notification.

Examples of the load states in (3) may include a resource state and afree resource state. The resource state may be indicated by a radioresource. Consequently, the CU can communicate with the UE using the DUthat can reserve a wide frequency band in a radio channel.Alternatively, a buffer volume may be used. The buffer volume may be,for example, (a) a buffer volume in the RLC layer, (b) a buffer volumein HARQ, or a sum volume of (a) and (b). Consequently, the CU can selecta DU while avoiding congestion of the user data.

Alternatively, another example of the load state in (3) may be thenumber of connected UEs. Since the CU can, for example, allocatehigh-volume communication to a DU with less number of connected UEs, DUscan be efficiently allocated.

With application of (3), the CU can select a DU in consideration ofcommunication states with the other UEs.

Examples of the load state of (4) may include a buffer volume. Thebuffer volume may be, for example, (a) a buffer volume in a new layerhigher than the PDCP in a U-Plane under NR, (b) a buffer volume in thePDCP layer, or a sum volume of (a) and (b). The buffer volume may be avalue for each corresponding UE or a sum value of the corresponding UEs.This enables flexible control including, for example, application of aplurality of use DUs when a buffer volume is larger or application of asingle use DU when the buffer volume is smaller.

The CU may request information of (1) to (3) and (5) from each DU. TheDUs may notify the CU of the information of (1) to (3) and (5). The DUsmay give the notifications periodically or in response to the requestsfrom the CU. Alternatively, the DUs may give the notifications whensatisfying a predefined condition. The condition may be determined in astandard or notified from the CU to the DUs.

When a use DU for communication between the CU and the UE (hereinaftermay be referred to as a “use DU”) is determined using (1) to (5) above,a threshold may be set. The CU may determine the use DU using thethreshold. For example, when the received intensity of the uplinkreference signal received from the UE is higher than or equal to acertain value, the CU may determine that the DU is available forcommunication between the CU and the UE. Alternatively, for example,when the buffer volume in the DU is larger than or equal to a certainvalue, the CU may determine that the DU is available for communicationbetween the CU and the UE. This enables the CU to easily determine theuse DU.

An interface between the CU and the DU may be used for communicationbetween the CU and the DU. The interface between the CU and the DU maybe, for example, the Fs interface (see Non-Patent Document 7). The Fsinterface may be used when the CU requests the information of (1) to (3)and (5) from the DU or when the DU notifies the CU of the information of(1) to (3) and (5). The CU may request the information of (1) to (3) and(5) from the DU by piggybacking the request onto the user data orindependently. The DU may notify the CU of the information of (1) to (3)and (5) by piggybacking the notification onto the user data orindependently. The request or the notification may be piggybacked ontothe user data by, for example, inserting the request or the notificationinto a free space (a padding area) of the user data. Consequently, theoverhead such as a header can be reduced in the forwarding. Making therequest and the notification independently of the user data can expeditethe request and the notification.

According to the first embodiment, DUs to be candidates for the use DU(hereinafter may be referred to as “candidate DUs”) may be provided. Thecandidate DUs may be, for example, DUs whose PDCCHs are to be monitoredby the UE. The candidate DUs and the use DU may be determined in stages.For example, a use DU may be selected from among the candidate DUs.Consequently, the use DU can be flexibly set.

The candidate DUs may be all the DUs being served by the CU. Thisincreases the flexibility of communication. The candidate DUs may bepart of the DUs being served by the CU. Since the number of the DUswhose PDCCHs are to be monitored by the UE can be reduced, the powerconsumption of the UE can be reduced.

The CU may notify the UE of information indicating which DU should bethe use DU. The L1/L2 signaling may be used for the notification. TheL1/L2 signaling may be, for example, scheduling information, that is,downlink allocation information and uplink grant information. The CU maytransmit the L1/L2 signaling to the UE through all or one of thecandidate DUs. The CU may transmit the L1/L2 signaling to the UE throughthe use DU in the downlink communication. The CU may include, in theL1/L2 signaling, the downlink allocation information of the use DU, theuplink grant information, or both the downlink allocation informationand the uplink grant information. The UE may determine the use DU, usingthe presence or absence of the scheduling information. Consequently, theCU can flexibly change the use DU, and promptly notify the UE ofinformation on the use DU.

The CU may determine the candidate DUs. When the CU determines thecandidate DUs, the CU may use information identical to the informationof (1) to (5) to be used for determining the use DU. The information tobe used by the CU for determining the candidate DUs may be identical toor different from that to be used by the CU for determining the use DU.Consequently, the complexity incurred when the CU determines thecandidate DUs and the use DU can be avoided.

A DU itself may determine a DU (a use DU) to be used for communicationbetween the CU and the UE, and the candidate DUs. The DU may request theCU to use its own DU for the communication between the CU and the UE.The CU may accept or reject the request. Making such a determination bythe DU itself can reduce the amount of processing for determining theuse DU by the CU. Since the DU need not notify the CU of a measurementresult, the amount of signaling through the interface between the CU andthe DU, for example, the Fs interface can be reduced.

Examples of (1) to (5) below are disclosed as information to be usedwhen the DU makes the determination:

(1) a measurement result obtained by its own DU, for example, ameasurement result of an uplink signal;

(2) a measurement result obtained by the UE, for example, a measurementresult of a downlink signal;

(3) a load state of its own DU;

(4) a load state of the CU; and

(5) combinations of (1) and (4) above.

In (1), the DU may use information identical to the aforementionedinformation of (1) and (2) to be used by the CU for determining the useDU. This can produce the same advantages as those when the CU uses theinformation of (1) or (2) for determining the use DU.

In (3), the DU may use information identical to the aforementionedinformation of (3) to be used by the CU for determining the use DU.Since this eliminates the need for the DU to notify the CU of a loadstate of its own DU, the amount of signaling through the interfacebetween the CU and the DU, for example, the Fs interface can be reduced.

In (4), the DU may use information identical to the aforementionedinformation of (4) to be used by the CU for determining the use DU.Consequently, the DU can flexibly change the use DU according to thecommunication volume necessary between the CU and the UE.

The DU may request the information of (4) from the CU. The CU may notifythe DU of information of (4). The CU may give the notificationperiodically or in response to the request from the DU. Alternatively,the CU may give the notification when satisfying a predefined condition.The condition may be determined in a standard or notified from the CU tothe DU. Consequently, the amount of signaling required for making thenotification from the CU to the DU can be optimized.

A threshold for determining the use DU using (1) to (4) above may beset. The DU may determine the use DU using the threshold. For example,when the received intensity of the uplink reference signal received fromthe UE is higher than or equal to a certain value, the DU may determinethat its own DU is available for communication between the CU and theUE. Alternatively, for example, when the buffer volume in its own DU islarger than or equal to a certain value, the DU may determine that itsown DU is available for communication between the CU and the UE. Thisenables the DU to easily determine the use DU.

The UE may determine the use DU and the candidate DUs. The UE mayrequest the CU to regard the DU determined by itself as a use DU or acandidate DU. A scheduling request from the UE to the CU may include therequest. The CU may accept or reject the request. The determination onthe use DU by the UE enables, for example, prompt switching of the useDU following the movement of the UE.

When the UE determines the use DU, the UE may use the measurement resultobtained by its own UE. The measurement result may be identical to theaforementioned information of (2) to be used by the CU for determiningthe use DU. This can produce the same advantages as those when the CUuses the information of (2) for determining the use DU. Moreover, theamount of signaling through the interfaces between the CU and the DU,for example, the Fs interface and a radio interface can be reduced.

The use DU in the uplink communication may be identical to or differentfrom that in the downlink communication. The candidate DUs in the uplinkcommunication may be identical to or different from those in thedownlink communication. The use of the same DU makes the control in theCU and the UE easy. Since the use of the different DUs enables selectionof appropriate DUs according to a channel state in each of the uplinkcommunication and the downlink communication, the communication capacitycan be increased and the low latency and the high reliability can beensured. The entities that determine the candidate DUs and the use DUmay be different between the uplink communication and the downlinkcommunication. For example, the CU may determine the candidate DUs andthe use DU in the uplink communication, and the UE may determine thecandidate DUs and the use DU in the downlink communication. Since theuse DU can be controlled according to a channel state that can beactually measured by the receiver, the reliability of communication canbe enhanced. Alternatively, for example, the UE may determine thecandidate DUs and the use DU in the uplink communication, and the CU maydetermine the candidate DUs and the use DU in the downlinkcommunication. Since the measurement result need not be fed back througha radio interface, the amount of signaling through the radio interfacecan be reduced. Alternatively, for example, the CU may determine thecandidate DUs both in the uplink communication and the downlinkcommunication, the UE may determine the use DU in the downlinkcommunication, and the CU may determine the use DU in the uplinkcommunication. Since the use DU can be controlled according to a channelstate that can be actually measured by the receiver, the reliability ofcommunication can be enhanced. Moreover, the collective determination ofthe candidate DUs by the CU enables prompt determination of the use DUs.

A method for making the use DUs different between the uplinkcommunication and the downlink communication is disclosed.

In the downlink communication, the CU may include, in the L1/L2signaling for the use DU, the scheduling information of the use DU, thatis, the downlink allocation information. The UE may determine, as theuse DU, the DU including the downlink allocation information in theL1/L2 signaling. Consequently, the UE can promptly determine the use DUin the downlink communication.

In the uplink communication, the CU may combine, with the L1/L2signaling from the DU to the UE, the uplink grant information andinformation on the use DU in the uplink communication, and notify theresulting information. The CU may give the notification using one of thecandidate DUs in the downlink communication. The UE may determine theuse DU in the uplink communication, using the L1/L2 signaling.Consequently, the UE can promptly determine the use DU in the uplinkcommunication.

In the uplink communication, the UE may transmit data to a plurality ofDUs simultaneously or with different timings. The CU may notify the UEof whether the data should be transmitted simultaneously or withdifferent timings. The CU may give the notification via the L1/L2signaling, via the MAC signaling, or semi-statically via the RRCsignaling. This can increase the flexibility in the uplinkcommunication.

A primary DU and a secondary DU may be provided according to the presentinvention. Since this facilitates the selection of the use DU in the CUand the UE, the amount of processing in the CU and the UE and the amountof signaling can be reduced. The DUs other than the primary DU may besecondary DUs. The number of the primary DUs may be one or more. All ora part of the DUs being served by the CU may be primary DUs.

The primary DU may be, for example, a use DU or a candidate DU intransmission of C-Plane data. This facilitates the processing forforwarding the C-Plane data. Alternatively, the primary DU may be a DUto which data is conducted more preferentially than to the secondary DUin both the C-Plane and the U-Plane. Consequently, the amount ofprocessing for determining the use DU can be reduced in both the C-Planeand the U-Plane.

The CU may determine which one of the DUs being served by the CU shouldbe the primary DU. This facilitates the control over the DUs in thewhole communication system.

The UE may determine which one of the DUs being served by the CU shouldbe the primary DU. Due to this, the UE need not notify the CU of ameasurement result, so that the amount of signaling can be reduced.Among the DUs being served by the CU, a DU to be the primary DU may bedifferent for each UE. Since the optimal conductive path can be builtfor each UE, the reliability and the flexibility in a communicationsystem can be increased. The CU and the UE may determine different DUsas the primary DUs in each of the uplink communication and the downlinkcommunication. Consequently, a conductive path can be flexibly builtaccording to differences in channel state between the uplinkcommunication and the downlink communication, and the reliability ofcommunication can be enhanced.

The CU may determine the primary DU using information on the performanceof the DUs being served thereby. The performance may be indicated by acommunication range, a use frequency bandwidth, a buffer volume, or acommunication capacity between the CU and the DU. This enables efficientcommunication in a communication system. For example, since the CU canperform C-Plane communication with many UEs, using a DU with a widecommunication range as the primary DU, the C-Plane communication can beefficiently performed.

When the CU determines the primary DU using information on theperformance of the DUs, the DU may notify the CU of information on theperformance of its own DU. The DU may give the notification when the DUstarts the connection with the CU. The CU may request, from the DU, theinformation on the performance of the DU. The CU may make the requestwhen the DU starts the connection with the CU. Soon after the DU startsthe connection with the CU, the CU can properly determine the primaryDU.

The request and the notification may be made upon setting up theinterface between the CU and the DU, for example, the Fs interface. Therequest and the notification may be included in signaling for setting upthe Fs interface. Consequently, the amount of signaling between the CUand the DU can be reduced.

The request and the notification may be made when the performance of theDU is updated. Consequently, the CU and the UE can properly determinethe primary DU with reflection of the updated performance of the DU.

The request and the notification may be made upon start of communicationwith the UE. Consequently, the primary DU can be flexibly determined foreach UE.

The CU and/or the UE may determine a DU with superior communicationquality with the UE as the primary DU. Consequently, the reliability ofcommunication can be enhanced. The communication quality may beindicated by a received intensity (for example, an RSRP) or asignal-to-interference-plus-noise ratio. Another indicator, for example,an RSRQ may be used. A certain threshold may be set to the communicationquality, and a DU with superior communication quality relative to thethreshold or a DU with communication quality higher than or equal to thethreshold may be determined as the primary DU. Alternatively, theprimary DUs may be a predetermined number of DUs from the DU with thehighest communication quality. The threshold and/or the predeterminednumber may be determined in a standard, or determined by the CU. The CUmay notify the UE of the threshold and/or the predetermined number. TheCU may give the notification via the RRC signaling, the MAC signaling,or the L1/L2 signaling.

The CU and/or the UE may determine the DU on which the UE has beencamping as the primary DU. This facilitates the control over the DU.

Alternatively, the CU and/or the UE may determine, as the primary DU, aDU to which the UE is RRC-connected. This facilitates the control overthe DU.

Alternatively, when determining the primary DU, the CU and/or the UE mayuse the information of (1) to (4) for the CU to determine which one ofthe DUs being served thereby is used for performing communication. Sincethis can integrate the determination on the use DU with thedetermination on the primary DU, the communication system can be easilydesigned, and the amount of processing can be reduced.

The CU and/or the UE may fixedly determine the primary DU. In otherwords, the CU and/or the UE may continuously use, as the primary DU, theprimary DU determined once. Consequently, the CU easily controls the DU.

Alternatively, the CU and/or the UE may variably determine the primaryDU. In other words, the CU and/or the UE may change the primary DUdetermined once. This can increase the flexibility in the communicationsystem.

The CU may forward the duplicated packet to each DU through theinterface between the CU and the DU, for example, the Fs interface.Similarly, each of the DUs may forward the received duplicated packet tothe CU through the interface between the CU and the DU, for example, theFs interface.

The CU may forward the different numbers of duplicated packets to thedifferent DUs. For example, when the DUs #1 and #2 exist under the CU,the CU may duplicate a packet into three, and forward two of the packetsto the DU #1 and the remaining one packet to the DU #2. Consequently,the reliability in the downlink communication can be enhanced. Forexample, when a channel state between the DU #1 and the UE is inferior,which may cause excess of the maximum number of retransmissions in HARQ,the CU forwards the two packets to the DU #1 as previously described.Even when packet loss on one of the packets occurs due to the excess ofthe maximum number of retransmissions in HARQ, the DU #1 can transmitthe other packet to the UE.

The UE may forward the different numbers of duplicated packets tolower-layer entities that correspond to the different DUs. For example,when the UE communicates with a CU having two DUs being served thereby(the DUs #1 and #2), the PDCP layer of the UE may duplicate a packetinto three, and forward two of the packets to an RLC entitycorresponding to the DU #1 and the remaining one packet to an RLC entitycorresponding to the DU #2. Consequently, the reliability in the uplinkcommunication can be enhanced.

When the CU determines the number of duplicated packets to be forwardedto the DUs, the CU may use information identical to the information fordetermining the use DU by its own CU. When the UE determines the numberof duplicated packets to be forwarded to lower layers in thecorresponding DUs, the UE may use information identical to theinformation for determining the use DU by its own UE. Consequently, thecomplexity in building a communication system can be avoided.

The CU and/or the UE may give the duplicated packets the same PDCPsequence number. Consequently, the UE and/or the CU to be a receivereasily detects redundant packets.

Alternatively, the CU and/or the UE may give the duplicated packetsdifferent PDCP sequence numbers. This can avoid the complexity indesigning, in the CU and/or the UE to be a transmitter, a PDCP headerassigner and a corresponding RLC forwarding unit.

The CU and/or the UE may give the duplicated packets different serialnumbers. For example, when the PDCP sequence numbers range from #1 to#7, the CU and/or the UE may give the PDCP sequence numbers #4 and #5 totwo packets that are duplicates of a packet, and give the PDCP sequencenumbers #6 and #7 to two different packets that are duplicates ofanother packet. This can avoid the complexity in designing the PDCPheader assigner.

The CU and/or the UE may provide an identifier indicating a duplicatedpacket, and transmit the identifier to the receiver. The identifier maybe included in, for example, a PDCP header. Consequently, the receivercan easily identify the identical packets.

Alternatively, the CU and/or the UE may give the respective duplicatedpackets PDCP sequence numbers with branch numbers. For example, the CUand/or the UE may give the PDCP sequence numbers #3-1 and #3-2 to twoduplicated packets. Consequently, the receiver can easily identify theidentical packets without adding the PDCP headers.

Alternatively, even if branch numbers are adopted, there may be a packetto which no branch number is given. For example, the PDCP sequencenumbers #3 and #3-1 may be given to two duplicated packets.Consequently, the number of bits of the PDCP sequence numbers can bereduced.

The UE and/or the CU may discard PDCP-PDUs redundantly received. The UEand/or the CU may receive the PDCP-PDU on a first-come, first-servedbasis. In other words, the UE and/or the CU may discard the PDCP-PDUreceived later. Consequently, the latency can be reduced. The UE and/orthe CU may immediately forward the received PDCP-PDU to an upper layer.Consequently, the latency can be reduced.

A priority may be assigned to each DU to be a conductive path betweenthe CU and the UE. The UE and/or the CU may preferentially receive thePDCP-PDU with the higher priority. For example, the UE and/or the CU maypreferentially receive the PDCP-PDU received through a DU with a lowlatency in a link between the CU and the DU. When a difference inlatency in the links between the CU and the DU is significant and whenthe reliability in one of the links is low, this facilitates the processof receiving the PDCP-PDU.

The CU or the UE may give the priority of each DU. Examples of (1) to(7) below are disclosed as information to be used when the CU and/or theUE determines the priority:

(1) the latency in the link between the CU and the DU;

(2) the reliability of the link between the CU and the DU;

(3) the propagation latency between the DU and the UE;

(4) the performance of the DU;

(5) a load state of the DU;

(6) information indicating the primary DU; and

(7) combinations of (1) and (6) above.

In (1), for example, assignment of a higher priority to a DU with alower latency in the link between the CU and the DU enables the CUand/or the DU to easily perform the process of receiving the PDCP-PDU.

In (2), for example, a packet loss rate in the link may be used.Consequently, for example, assignment of a higher priority to a DU in alink with a lower packet loss rate can enhance the reliability ofcommunication between the CU and the UE.

In (3), for example, assignment of a higher priority to a DU with alower propagation latency enables the CU and/or the DU to easily performthe process of receiving the PDCP-PDU.

In (4), for example, a time from receipt of a PDCP-PDU from an upperlayer to transmission of the PDCP-PDU to a radio interface may be used.Alternatively, a time from receipt of data from a radio interface totransmission of the data to the CU as the PDCP-PDU may be used.Consequently, for example, assignment of a higher priority to a DU withhigher performance enables the CU and/or the DU to easily perform theprocess of receiving the PDCP-PDU.

In (5), for example, a free resource state may be used. The freeresource state may be indicated by, for example, a buffer volume in theDU or the radio resource in the DU. Consequently, for example,assignment of a higher priority to a DU with a large free resource canincrease a transmission rate as well as facilitating the process ofreceiving the PDCP-PDU in the CU and/or the DU.

Alternatively, the number of UEs communicating using the DU may be usedin (5). For example, a higher priority may be assigned to a DU with lessnumber of the UEs. Since this can shorten the time for waiting forscheduling the user data, the latency involved in the communication canbe reduced.

In (6), the amount of processing for determining a priority by the CUcan be reduced.

The CU and the UE may measure the latency in the link between the CU andthe UE, Here, the CU and the UE may measure a relative value of thelatencies in different links passing through different DUs. Sincemeasurement of the relative value eliminates the need for thetransmitter to give a time stamp, the measurement process isfacilitated.

The measurement may be performed through the interface between the CUand the DU, for example, each of the Fs interface and a radio interfacebetween the DU and the UE. This facilitates the measurement process.

The CU and the UE may use data for measurement to perform themeasurement. The data for measurement may be, for example, the RRCsignaling or a PDCP Status PDU. The data for measurement may include atime stamp. The UE may find the latency in the link, using an arrivaltime of the data for measurement and the time stamp.

Consequently, the absolute value of the latency in the link can bemeasured. Furthermore, the amount of processing in the DU for measuringthe latency in the link between the CU and the UE can be reduced.

Alternatively, the CU and the UE may use the user data. Consequently,the overhead involved in the measurement can be reduced. When the CU andthe UE perform the measurement using the user data, the transmitter mayuse information indicating that the user data is to be measured. The CUmay notify the UE of the information in advance, or an identifierindicating the information may be added to the user data.

When the CU and the UE perform the measurement, different signals may beused in the measurement through the interfaces between the CU and theDU, for example, the Fs interface and the radio interface. For example,pilot data with a time stamp may be used through the Fs interface,whereas an uplink reference signal or a downlink reference signal may beused through the radio interface. Consequently, the overhead involved inthe measurement can be reduced.

The UE may notify the CU of a measurement result of the latency in alink passing through each DU. The CU may determine a priority of thelink passing through each DU, using the measurement result. The UE mayuse the RRC signaling for the notification. Consequently, the processingtime in the DU, for example, the time for decoding data can be reduced.Alternatively, the UE may use the PDCP Status PDU. This allows thenotification with a smaller overhead. Alternatively, differentsignalings may be used between a notification from the UE to the DU anda notification from the DU to the CU. For example, the UE may notify theDU via the L1/L2 signaling or the MAC signaling. The L1/L2 signalingallows a prompt notification. The MAC signaling allows a notificationwith less number of symbols through application of the multi-levelmodulations. Further, the DU may notify the CU through the interfacebetween the CU and the DU, for example, using a control signal of the Fsinterface. The control signal may be piggybacked onto the user data ortransmitted independently of the user data. The use of differentsignalings between the notification from the UE to the DU and thenotification from the DU to the CU reduces the overhead involved in thefeedback.

The UE may notify the CU of a result of the averaging procedure or thefiltering procedure on measurement values of the latency. The UE maynotify the result of the averaging procedure or the filtering procedure,in the same method for notifying the measurement result of the latencyfrom the UE to the CU. This can suppress the influence of fluctuationsin the measurement value, and reduce the amount of signaling in thenotification.

A parameter to be used for the averaging procedure or the filteringprocedure may be determined in a standard or by the CU. The CU maynotify the UE of the parameter. This enables flexible measurementaccording to a state of the communication system.

The CU may notify the UE of a measurement result of the latency in alink passing through each DU. The UE may determine a priority of thelink passing through each DU, using the measurement result. The CU mayuse the RRC signaling for the notification. Consequently, the processingtime in the DU, for example, the time for decoding data can be reduced.Alternatively, the CU may use the PDCP Status PDU. This allows thenotification with a smaller overhead. Alternatively, differentsignalings may be used between a notification from the CU to the DU anda notification from the DU to the UE. For example, the CU may notify theDU through the interface between the CU and the DU, for example, using acontrol signal of the Fs interface. The control signal may bepiggybacked onto the user data or transmitted independently of the userdata. The control signal may be piggybacked onto the user data by, forexample, inserting the control signal into a free space (a padding area)of the user data. The DU may notify the UE via the L1/L2 signaling orthe MAC signaling. The L1/L2 signaling allows a prompt notification. TheMAC signaling allows a notification with less number of symbols throughapplication of the multi-level modulations. The use of differentsignalings between the notification from the CU to the DU and thenotification from the DU to the UE reduces the overhead involved in thefeedback.

The CU may notify the UE of the result of the averaging procedure or thefiltering procedure on measurement values of the latency. The CU maynotify the result of the averaging procedure or the filtering procedure,in the same method for notifying the measurement result of the latencyfrom the CU to the UE. This can suppress the influence of fluctuationsin the measurement value, and reduce the amount of signaling in thenotification.

The parameter to be used for the averaging procedure or the filteringprocedure may be determined in a standard or by the CU. The CU maynotify the UE of the parameter. This enables flexible measurementaccording to a state of the communication system.

The CU may determine the priority. The CU may notify the UE of thepriority. Consequently, the CU can optimize the communication rate inview of communication states of the other UEs. The notification mayinclude an identifier indicating a priority and an identifier indicatinga DU. The notification may indicate a priority of each DU. Consequently,the CU can notify the UE with less amount of signaling. Alternatively,the notification may indicate a ratio of amounts of forwarded duplicatedpackets for each DU. Since this enables flexible specification of amethod for forwarding duplicated packets, the reliability ofcommunication can be efficiently increased.

The CU may use the PDCP control PDU for the notification. This allowsthe notification with a smaller overhead. Alternatively, the RRCsignaling may be used for the notification. Consequently, a large amountof information can be notified.

The UE may determine the priority. The UE may use the measurement resultobtained by its own UE for the determination. The UE may notify the CUof the priority. Since this eliminates the need for feeding back themeasurement result, the amount of signaling can be reduced.

In the communication between the CU and the UE, the priority of the useDU may be changed between the uplink and the downlink. The priority canbe flexibly controlled according to a channel state in each of theuplink communication and the downlink communication. Different entitiesmay determine the priorities of the use DUs in the uplink and thedownlink. For example, the CU may determine the priority in the uplink,whereas the UE may determine the priority in the downlink. Consequently,the amount of signaling can be reduced in the measurement and in thefeedback of the measurement result.

In the communication between the CU and the UE, the PDCP layer in thereceiver may generate PDCP-SDUs using the duplicated PDCP-PDUs, andforward the PDCP-SDUs to the upper layer. The upper layer may be, forexample, an application layer or the RRC. The number of the PDCP-SDUs tobe forwarded may be one or more. In other words, the receiver need notnecessarily generate the PDCP-SDUs to correspond to all the duplicatedPDCP-PDUs. This can reduce the latency in the receiver. The receiver mayreceive the duplicated PDCP-PDUs using a reordering timer. This enablessmooth operations in the system.

In the communication between the CU and the UE, the PDCP layer in thereceiver may determine through which DU the PDCP-PDU has been received.Each RLC entity in the receiver may forward an identifier indicating itsown entity to the PDCP layer, together with the PDCP-PDU. Alternatively,an identifier indicating a DU used in the communication may be used.This enables the receiver to easily determine the priority of the DU.Since this eliminates the need for notifying the identifier of the DUthrough a radio interface, the amount of signaling through the radiointerface can be reduced.

The PDCP layer in the receiver may remove the identifier in generatingthe PDCP-service data unit (PDCP-SDU) from the PDCP-PDU. Consequently,the amount of data processing in the upper layer, for example, theapplication layer can be reduced. Alternatively, the PDCP layer in thereceiver may maintain the identifier in generating the PDCP-SDU and thenforward the PDCP-SDU to the upper layer, for example, the RRC. Thisfacilitates the control over the use DU in the upper layer, for example,the RRC. The PDCP layer in the receiver may forward, to the upper layer,for example, to the application layer, the PDCP-SDUs from which theidentifier has been removed, and forward information on the removedidentifier to another upper layer, for example, to the RRC.Consequently, the amount of processing in, for example, the applicationlayer and the RRC can be reduced.

The transmitter may assign an identifier in the communication betweenthe CU and the UE. The RLC layer in the receiver may forward theidentifier to the PDCP layer in the receiver. Consequently, the amountof processing in the RLC layer in the receiver can be reduced. The PDCPlayer in the receiver may forward, to the upper layer, for example, tothe application layer, data from which the identifier has been removed.Thus, the amount of processing in the application layer can be reduced.The PDCP layer in the receiver may forward information on the identifierto another upper layer, for example, to the RRC. This facilitates thecontrol over the use DU in the RRC.

The CU may obtain information on the packet that has been received fromeach DU. The information may be, for example, a packet loss rate or thelatency in the packet. Consequently, the CU can easily control the useDU.

Alternatively, the UE may obtain information on the packet that has beenreceived from each RLC entity. The information may be, for example, apacket loss rate or the latency in the packet. Consequently, the UE caneasily control the use DU.

The identifier indicating the DU may be an IP address of the DU. Sincethis enables use of a configuration of an interface between basestations as it is, the complexity in designing the CU and the DU can beavoided. The overhead of the processing in the RLC layer can be reduced.

The identifier indicating the DU (hereinafter may be referred to as a“DU-ID”) may be given a unique identification among the DUs being servedby the CU. This enables identification of the DU with less number ofbits.

Alternatively, the DU-ID may be given a unique identification amongsurrounding gNBs. Since the CU in the master base station can identifythe DU in the secondary base station in the combination of the DC or theMC with the CU-DU split, the flexibility of the configuration betweenthe CU and the DU can be increased. The surrounding gNBs may be gNBs inthe same tracking area. This can prevent dual management of thesurrounding gNBs for identification of the DU and the gNBs in thetracking area, and avoid the complexity in the device. Alternatively,the surrounding gNBs may be gNBs to be connected to the same mobilitymanagement entity (MME). This facilitates the identification of the DUin the mobility between the gNBs.

Alternatively, the DU-ID may be given a unique identification betweenthe master base station and the secondary base station. Even when themaster base station and the secondary base station straddle a boundaryof the surrounding gNBs, the master base station and the secondary basestations being served thereby can easily identify the DU.

Alternatively, the identifier given a unique identification among theDUs being served by the CU of the gNB may be combined with the gNBidentifier for use. Consequently, the DU can be easily identified fromthe other gNBs. A Physical Cell Identity (abbreviated as PCI) may beused instead of the gNB identifier. Since this facilitates theidentification of the cell and the DU, the system is easily controlled.

Alternatively, unique identifiers may be given to all the DUs in thecommunication system. This facilitates the identification of the DUs inthe communication system.

The identifiers of the DUs may be renumbered. This enables reduction inthe number of bits for identifying the DUs through, for example,signaling. For example, a table may be used for the renumbering. Thetable should include the original identifier of the DU and therenumbered identifier. This facilitates the association between theoriginal identifier of the DU and the renumbered identifier.

The high-level network device may renumber the identifiers. This enablesmanagement of the DUs across a plurality of gNBs with less number ofbits.

Alternatively, the master base station may renumber the identifiers.This enables management of the DUs in the configuration of the DC or theMC with less number of bits.

The master base station may request the secondary base station to notifyidentifiers of the DUs being served by the secondary base station. Thesecondary base station may notify the master base station of theidentifiers of the DUs being served by its own secondary base station.Since the secondary base station has only to notify the identifiers ofthe DUs with the timing necessary for the master base station, theamount of signaling can be reduced. Alternatively, the secondary basestation may notify the master base station of the identifiers of the DUsbeing served by its own base station when the configuration of the DUsbeing served by its own base station is changed. Consequently, themaster base station can perform control promptly reflecting the changein the configuration of the DUs being served by the secondary basestation.

Each base station may renumber the identifiers. This can reduceoperations of inquiring the mapping of the identifiers between basestations or between a base station and the high-level network device.Consequently, the amount of signaling can be reduced.

The CU and the UE may share information on the renumbering. Theinformation may be information obtained by combining an identifier ofthe DU to be renumbered and the renumbered identifier of the DU. Theamount of signaling on the information of the DU can be reduced in thecommunication between the CU and the UE.

The CU may notify the UE of the information. The notification may begiven via the RRC signaling. Consequently, the amount of signalingprocessing in the use DU can be reduced.

Alternatively, the notification between the CU and the DU may beperformed through the interface between the CU and the DU, for example,using a control signal of the Fs interface. The control signal may bepiggybacked onto the user data or transmitted independently of the userdata. The control signal may be piggybacked onto the user data by, forexample, inserting the control signal into a free space (a padding area)of the user data. Consequently, the overhead involved in transmission ofthe control signal can be reduced. The notification between the DU andthe UE may be performed via the L1/L2 signaling. This enables a promptnotification from the DU to the UE. Alternatively, the MAC signaling maybe used for the notification. This allows the notification with lessnumber of symbols through application of the multi-level modulations,and increase in the reliability with the HARQ retransmission control.Alternatively, the same signaling method as that for carrier aggregationmay be used. Since this can integrate the signaling methods, thecomplexity in system design can be avoided.

The CU and the DU may share the information on the renumbering. The CUand the DU may mutually notify information necessary for the renumberingto share the information. The notification may be given through theinterface between the CU and the DU, for example, the Fs interface. TheDU may notify the CU of, for example, an identifier of its own DU as theinformation necessary for the renumbering. The CU may notify the DU ofthe renumbered identifier of the DU. This enables reduction in thenumber of bits required for notifying the identifier of the DU throughthe interface between the CU and the DU, for example, the Fs interface.

The high-level network device and the CU may share the information onthe renumbering. The high-level network device and the CU may mutuallynotify information necessary for the renumbering to share theinformation. An interface between the high-level network device and theCU may be used for the notification. The CU may notify the high-levelnetwork device of, for example, the identifiers of the DUs being servedthereby as the information necessary for the renumbering. Theidentifiers of the DUs may be identifiers combined with the identifiersof the gNBs. The high-level network device may notify informationobtained by combining an identifier of the DU to be renumbered and therenumbered identifier of the DU. Consequently, the interface between thehigh-level network device and the CU can reduce the amount of signalingfor the information on the DUs.

The CU and/or the UE may share a buffer in the PDCP layer between theDUs. Consequently, the buffer volume can be saved. Alternatively, thebuffer in the PDCP layer may be reserved for each DU. Consequently, evenin the presence of the DU with communication delay, the latency can bereduced because another DU can continue the communication.

FIG. 8 illustrates, in the downlink communication, a configuration ofthe CU that duplicates a packet in the PDCP layer and forwards theduplicated packets to a plurality of DUs, the DUs, and the UE thatdetects redundant packets. In FIG. 8, a CU 801 communicates with a UE804 using a DU #1 (may be referred to as a “DU 802”) and a DU #2 (may bereferred to as a “DU 803”).

In FIG. 8, a high-level network device 805 forwards a packet 806 to aNew AS layer 807 in the CU 801. The New AS layer is a layer having afunction of mapping a QoS flow to a Data Radio Bearer (abbreviated asDRB) in the U-Plane under NR (see Non-Patent Document 11). The New ASlayer 807 generates a PDCP-Service Data Unit (PDCP-SDU) 808 using thepacket 806, and forwards the PDCP-SDU to a PDCP layer 809.

In FIG. 8, the PDCP layer 809 duplicates the PDCP-SDU 808 into two,assigns a PDCP header 810 to one of the duplicated SDUs to generate aPDCP-PDU #1 (may be referred to as a PDCP-PDU 812), and assigns a PDCPheader 811 to the other duplicated SDU to generate a PDCP-PDU #2 (may bereferred to as a PDCP-PDU 813). Although the PDCP headers 810 and 811include information on the same sequence number #n in FIG. 8, they mayinclude information on different sequence numbers. For example,assignment of serial sequence numbers to the PDCP headers 810 and 811facilitates design of the sequence number assigner in the PDCP layer.

In FIG. 8, the PDCP layer 809 forwards the PDCP-PDU #1 to an RLC layer816 of the DU #1 through an Fs interface 814. Furthermore, the PDCPlayer 809 forwards the PDCP-PDU #2 to an RLC layer 817 of the DU #2through an Fs interface 815.

In FIG. 8, the DU #1 transmits the PDCP-PDU #1 received in the RLC layer816 to a DU #1-corresponding entity 818 in the UE 804. The DU #2transmits the PDCP-PDU #2 received in the RLC layer 817 to a DU#2-corresponding entity 819 in the UE 804. An RLC layer 820 forwards thereceived PDCP-PDU #1 to a PDCP layer 822. An RLC layer 821 forwards thereceived PDCP-PDU #2 to the PDCP layer 822.

In FIG. 8, the PDCP layer 822 detects a redundant packet. In the exampleof FIG. 8, the PDCP layer 822 detects that the PDCP-PDU #1 and thePDCP-PDU #2 are identical, and removes the PDCP-PDU #2. The PDCP layer822 removes the PDCP header 810 from the PDCP-PDU #1 to obtain thePDCP-SDU 808, and forwards the PDCP-SDU 808 to a New AS layer 823.Although the PDCP layer 822 removes the PDCP-PDU #2 in the example ofFIG. 8, it may remove the PDCP-PDU #1. In such a case, the PDCP layer822 removes the PDCP header 811 from the PDCP-PDU #2 to obtain thePDCP-SDU 808, and forwards the PDCP-SDU 808 to the New AS layer 823.

In FIG. 8, the New AS layer 823 restores the packet 806 using thePDCP-SDU 808, and forwards the packet 806 to an upper layer 824.

In the communication between the CU and the UE, a bearer that passesthrough all the DUs being served by the CU may be established. The CUmay notify the UE of change in the setting on the communication witheach of the DUs, in the mobility of the UE between the DUs. Since thiseliminates the need for changing the bearer even when the UE movesbetween the DUs, the amount of signaling can be reduced.

The CU may reserve radio resources of all the DUs being served thereby.The CU may reserve the buffer volumes of all the DUs being servedthereby. The radio resources and/or the buffer volumes may be reservedwhen the CU newly sets a bearer. The radio resources and/or the buffervolumes may be reserved when the CU changes the setting of the bearer.Consequently, even when the UE performs mobility between the DUs, the UEcan continue the communication while the QoS (for example, bandwidthguarantees) requested for the bearer is satisfied.

The CU may reserve the radio resources of part of the DUs being servedthereby. The number of the DUs whose radio resources are reserved may beone or more. Since the radio resources to be reserved in establishingthe bearer can be saved, the efficient communication is possible.

The part of the DUs may be a predetermined number of DUs from the DUwith the highest measurement result. Consequently, the radio resourcescan be saved while the reliability of the communication is increased.

Alternatively, the part of the DUs may be DUs adjacent to the use DU inthe communication between the CU and the UE. Consequently, the radioresources to be reserved by the CU for the DUs can be further saved.

Alternatively, the part of the DUs may be DUs surrounding the use DU inthe communication between the CU and the UE. The surrounding DUs mayinclude the DUs adjacent to the use DU. This enables smooth mobilitybetween the DUs even upon sudden change in a radio channel state, andsaving of the radio resources.

FIG. 9 illustrates a configuration of a bearer that passes through allthe DUs being served by the CU. FIG. 9 illustrates an example where a CU900 has a DU #1, a DU #2, and a DU #3 (may be referred to as a DU 901, aDU 902, and a DU 903, respectively) being served thereby and where theCU 900 communicates with a UE 904.

In FIG. 9, a bearer 905 is established as the bearer that passes throughall the DUs being served by the CU. The bearer 905 terminates in a PDCPlayer 906 of the CU 901 and a PDCP layer 907 of the UE 904. The bearer905 passes through the RLC layer, the MAC layer and the PHY layerincluded in each of the DU #1, the DU #2 and the DU #3. The bearer 905also passes through a DU #1-corresponding entity 908, a DU#2-corresponding entity 909, and a DU #3-corresponding entity 910 of theUE 904.

Although FIG. 9 illustrates an example of the bearer that passes throughall the DUs being served by the CU, the bearer may be a bearer thatpasses through part of the DUs. The bearer may be configured by, forexample, candidate DUs in the communication with the UE. Alternatively,the bearer may be configured by the use DUs in the communication withthe UE. This can reduce reservation of the unnecessary resources.

The CU may request, from the DUs being served thereby, information onthe performance of the DUs. The CU may request the DUs being newlyserved by the CU, for example, the DUs being connected to the CU. Theinformation on the performance of the DUs may be, for example, a timefrom receipt of a PDCP-PDU from the upper layer to transmission of thePDCP-PDU to a radio interface. Alternatively, the information on theperformance of the DUs may be a time from receipt of data from a radiointerface to transmission of the data to the CU as the PDCP-PDU.Alternatively, the information on the performance of the DUs may be thebuffer volume or radio resources of the DUs. Consequently, the CU canappropriately change the setting of the bearer using the information onthe performance of the DUs, when adding a DU.

The DU may notify the CU of the information on the performance of itsown DU. The DU may give the notification when the DU is newly served bythe CU, for example, when the DU is connected to the CU. The informationmay be the same as that on the performance of the DUs. Consequently, thesame advantages can be produced.

The DU may request, from the CU, information on a communication state.The DU may make the request when the DU is newly served by the CU, forexample, when the DU is connected to the CU. The information on thecommunication state may be, for example, information on the UE withwhich the CU is communicating, information on a bearer used by the UE,or information on the setting of the bearer. Consequently, for example,start-up operations when the DU is connected can be expedited.

The CU may notify the DU of the information on the communication state.The CU may notify the DUs being newly served by the CU, for example, theDUs being connected to the CU. The information on the communicationstate may be the same as the aforementioned information. Consequently,the same advantages can be produced.

The request and the notification may be made upon setting up theinterface between the CU and the DU, for example, the Fs interface. Therequest and the notification may be included in the signaling forsetting up the Fs interface. Consequently, the amount of signalingbetween the CU and the DU can be reduced.

The request and the notification may be made when the performance of theDU is updated. Consequently, the CU and the UE can properly determinethe primary DU with reflection of the updated performance of the DU.

The request and the notification may be made upon start of communicationwith the UE. Consequently, the primary DU can be flexibly determined foreach UE.

In the communication between the CU and the UE, a split bearer may beestablished for the DUs to which the duplicated packets are to beforwarded (hereinafter may be referred to as a “split bearer between theDUs”). This enables saving of the resources of the DUs that are not usedfor conducting duplicated packets. The split bearer between the DUs neednot be released to the DU that has established the split bearer betweenthe DUs once. Examples of the case where the split bearer between theDUs need not be released include an occurrence of the mobility betweenthe DUs. The split bearer between the DUs may be released to the DU.Examples of the case where the split bearer between the DUs may bereleased include a connection release between the UE and the CU.Alternatively, the split bearer between the DUs may be released when theDU is in an overloaded state. Even when the DU previously used is reusedin the communication between the CU and the UE, the amount of signalingon the setting of the split bearer between the DUs can be reduced.

FIGS. 10 and 11 illustrate a sequence for starting transmission andreception of duplicated packets in the communication between the CU andthe UE. FIGS. 10 and 11 are connected across a location of a borderBL1011. FIGS. 10 and 11 illustrate an example of switching from thecommunication using the DU #1 to the communication with application ofduplicated packets using the DU #1 and the DU #2.

Steps ST1000, ST1001, and ST1002 of FIG. 10 indicate transmission andreception of the user data between a high-level NW device and the UE. InStep ST1000, the user data is transmitted and received between thehigh-level NW device and the CU. In Step ST1001, the user data istransmitted and received between the CU and the DU #1. In Step ST1002,the user data is transmitted and received between the DU #1 and the UE.

In Steps ST1003 and ST1004 of FIG. 10, the CU notifies the UE of thesignaling for RRC connection reconfiguration. The CU transmits thenotification to the DU #1 in Step ST1003, and the DU #1 transmits thenotification to the UE in Step ST1004. The notification includesinformation indicating addition of the DU #2, and information indicatingstart of packet duplication. The notification also includes informationon the RRC parameter for the DU #2-corresponding entity. Thenotification may include information on the RRC parameter for the DU#1-corresponding entity. The UE sets the RRC parameter for packetduplication and communication with the DU #2, using the informationreceived in Step ST1004.

In Steps ST1005 and ST1006 of FIG. 10, the UE transmits an RRCconnection reconfiguration complete notification to the CU. The UEtransmits the notification to the DU #1 in Step ST1005, and the DU #1transmits the notification to the CU in Step ST1006.

In Step ST1007 of FIG. 10, the CU notifies the DU #2 of an instructionfor starting communication. The instruction may include the RRCparameter. The instruction may also include information indicating thata packet is to be duplicated. In response to the instruction, the DU #2sets the RRC parameter for transmitting and receiving data to and fromthe UE.

In Step ST1008 of FIG. 10, the DU #2 notifies the CU of a response tothe instruction for starting communication. The response may includeinformation indicating completion of the setting in the DU #2.

In Steps ST1009 and ST1010 of FIG. 10, a random access procedure isperformed for communication between the UE and the CU through the DU #2.Signaling is performed between the DU #2 and the CU in Step ST1009, anda radio signal is transmitted and received between the UE and the DU #2in ST1010. Step ST1009 may represent a random access Msg3 from the DU #2to the CU, or information, from the DU #2 to the CU, indicating thecompletion of the random access procedure. Step ST1009 may alsorepresent an acknowledgement or a negative acknowledgement from the CUto the DU #2 in response to the information indicating the completion ofthe random access Msg3 or the random access procedure. Consequently, thereliability of the notification of the information indicating thecompletion of the random access Msg3 or the random access procedure canbe enhanced.

In ST1010 of FIG. 10, signaling necessary for starting transmission andreception of the duplicated packets using the DU #1 and the DU #2 iscompleted.

Steps ST1011 to ST1017 of FIG. 11 indicate transmission and reception ofthe downlink user data.

In Step ST1011 of FIG. 11, the high-level network device forwards theuser data to the CU. In Step ST1012, the CU duplicates the PDCP-PDUincluding the user data.

As illustrated in FIG. 11, the CU forwards the duplicated user data tothe DU #1 and the DU #2 in Steps ST1013 and ST1014, respectively. InStep ST1015, the DU #1 transmits the user data to the UE. In StepST1016, the DU #2 transmits the user data to the UE.

In Step ST1017 of FIG. 11, the UE detects redundancy in the user datareceived in Steps ST1015 and ST1016. The UE removes the user datareceived as redundant user data.

Steps ST1018 to ST1024 of FIG. 11 indicate transmission and reception ofthe uplink user data.

In Step ST1018 of FIG. 11, the UE duplicates the PDCP-PDU including theuser data.

As illustrated in FIG. 11, the UE transmits the duplicated user data tothe DU #1 and the DU #2 in Steps ST1019 and ST1020, respectively. InStep ST1021, the DU #1 forwards the user data to the CU. In Step ST1022,the DU #2 forwards the user data to the CU.

In Step ST1023 of FIG. 11, the CU detects redundancy in the user datareceived in Steps ST1021 and ST1022. The CU removes the user datareceived as redundant user data.

In Step ST1024 of FIG. 11, the CU forwards the received user data to thehigh-level network device.

Steps ST1018 to ST1024 indicating the transmission and reception of theuplink user data may be performed after or simultaneously with StepsST1011 to ST1017 indicating the transmission and reception of thedownlink user data. This increases the flexibility in transmission andreception of the user data.

The sequence for starting transmission and reception of duplicatedpackets between the CU and the UE in the example of FIGS. 10 and 11 doesnot require signaling for the high-level network device, for example,the path update procedure described in 10.1.2.8.1 of Non-Patent Document1.

In the sequence for starting transmission and reception of duplicatedpackets between the CU and the UE, for example, the CU may notify the DUthat is communicating with the UE of an instruction for changingcommunication. The DU may notify the CU of a response to the instructionfor changing communication. This enables flexible change in the RRCparameter when the DU is added.

In the instruction for changing communication, the CU may also transmit,to the DU, an identifier indicating no initialization of each of the RLClayer, the MAC layer, and the PHY layer during communication. This canprevent interruption of communication due to discard of data caused bythe initialization.

FIGS. 12 and 13 illustrate another sequence for starting transmissionand reception of duplicated packets in the communication between the CUand the UE. FIGS. 12 and 13 are connected across a location of a borderBL1213. In FIGS. 12 and 13, the same step numbers are assigned to thesame Steps as those in FIGS. 10 and 11, and the common descriptionthereof is omitted.

In Step ST1101 of FIG. 12, the CU notifies the DU #1 of an instructionfor changing communication. The instruction may include an instructionfor changing the RRC parameter. Further, an identifier indicating noinitialization of each layer should be added to the instruction. The DU#1 changes the RRC parameter for the UE, using the information receivedin Step ST1101.

In Step ST1102 of FIG. 12, the DU #1 notifies the CU of a response tothe instruction for changing communication. The response includesinformation indicating the completion of the setting in the DU #1.

The application of the sequence illustrated in FIGS. 12 and 13 enablesflexible setting of a communication channel according to addition of theDU.

In another sequence for starting transmission and reception ofduplicated packets between the CU and the UE, the RRC connectionreconfiguration from the CU to the UE may be performed after a responseto the instruction for starting communication. The RRC connectionreconfiguration may be performed upon receipt of an acknowledgement fromthe DU to be added in a response to the instruction for startingcommunication. Even upon failure of the processing by the DU in aresponse to the instruction for starting communication, re-execution ofthe RRC connection reconfiguration can be prevented. The time from theRRC connection reconfiguration to the random access procedure in the UEcan be shortened.

FIGS. 14 and 15 illustrate a sequence for starting transmission andreception of duplicated packets between the CU and the UE when the RRCconnection reconfiguration is performed after a response to theinstruction for starting communication. FIGS. 14 and 15 are connectedacross a location of a border BL1415. In FIGS. 14 and 15, the same stepnumbers are assigned to the same Steps as those in FIGS. 10 and 11, andthe common description thereof is omitted.

In Steps ST1201 and ST1202 of FIG. 14, the CU notifies the UE of thesignaling for the RRC connection reconfiguration. Steps ST1201 andST1202 may be identical to Steps ST1003 and ST1004 in FIG. 10,respectively. The UE sets the RRC parameter for packet duplication andcommunication with the DU #2, using the information received in StepST1202.

In Steps ST1203 and ST1204 of FIG. 14, the UE transmits an RRCconnection reconfiguration complete notification to the CU. Steps ST1203and ST1204 may be identical to Steps ST1005 and ST1006 in FIG. 10,respectively.

In Step ST1205 of FIG. 14, the CU notifies the DU #2 of completion ofthe RRC connection reconfiguration for the UE. The DU #2 starts therandom access procedure with the UE using the notification.

The sequence illustrated in FIG. 14 is different from that illustratedin FIG. 10 in that Step ST1201 is performed after Step ST1008. Even uponfailure of the processing by the DU in a response to the instruction forstarting communication, re-execution of the RRC connectionreconfiguration can be prevented. The time from the RRC connectionreconfiguration to the random access procedure in the UE can beshortened.

In an alternative example sequence for starting transmission andreception of duplicated packets in the communication between the CU andthe UE, the CU may instruct each DU and the UE without waiting forresponses from the DU and the UE. Consequently, the processing foradding the DU can be expedited.

FIGS. 16 and 17 illustrate a sequence for starting transmission andreception of duplicated packets between the CU and the UE when the CUinstructs each of DUs and the UE without waiting for responses from theDUs and the UE. FIGS. 16 and 17 are connected across a location of aborder BL1617. In FIGS. 16 and 17, the same step numbers are assigned tothe same Steps as those in FIGS. 10 and 11, and the common descriptionthereof is omitted.

In Steps ST1301 and ST1302 of FIG. 16, the CU notifies the UE of thesignaling for the RRC connection reconfiguration. Steps ST1301 andST1302 may be identical to Steps ST1003 and ST1004 in FIG. 10,respectively. The UE sets the RRC parameter for packet duplication andcommunication with the DU #2, using the information received in StepST1302.

In Step ST1303 of FIG. 16, the CU notifies the DU #2 of an instructionfor starting communication. Step ST1303 may be identical to Step ST1007in FIG. 10. The DU #2 sets the RRC parameter for transmitting andreceiving data to and from the UE, using the information received inStep ST1303.

In Step ST1304 of FIG. 16, the CU notifies the DU #1 of an instructionfor changing communication. Step ST1304 may be identical to Step ST1101in FIG. 12. The DU #1 changes the RRC parameter for the UE, using theinformation received in Step ST1304.

The orders of Steps ST1301, ST1303, and ST1304 in FIG. 16 may bedifferent from one another. This can bring flexibility in the operationsof the CU.

Since the CU issues the instructions without waiting for responses fromeach DU and the UE according to the sequence illustrated in FIGS. 16 and17, the processing for adding the DU can be expedited.

The CU may notify the DU of an instruction for stopping communication.The instruction for stopping communication may be issued upon stop ofpacket duplication. The DU may notify the CU of a response to theinstruction for stopping communication. This enables a smoothcontinuation of communication before and after the stop of packetduplication.

The CU may notify the UE of the RRC connection reconfiguration. The CUmay notify the RRC connection reconfiguration upon stop of packetduplication. The RRC connection reconfiguration may include informationindicating release of the DU. The UE may notify the CU of completion ofthe RRC connection reconfiguration. This enables a smooth continuationof communication before and after the stop of packet duplication. TheRRC connection reconfiguration may include information indicatingcancellation of the packet duplication. Alternatively, the RRCconnection reconfiguration need not include the information indicatingcancellation of the packet duplication. Consequently, the number of theuse DUs can be reduced while the packet duplication is maintained.

The CU may communicate the RRC connection reconfiguration to the UEusing the DU that is not released. Consequently, the CU can notify theRRC connection reconfiguration with high reliability. Alternatively, theCU may notify the RRC connection reconfiguration using the DU to bereleased. Consequently, the overhead in the DU that is not released canbe reduced.

FIG. 18 illustrates a sequence for stopping transmission and receptionof duplicated packets in the communication between the CU and the UE.FIG. 18 illustrates an example of switching from the communication withapplication of duplicated packets using the DU #1 and the DU #2 to thecommunication using the DU #1.

Steps ST1401 to ST1407 of FIG. 18 indicate user data communication usingpacket duplication. In Step ST1401, the high-level network devicetransmits and receives the user data to and from the CU. In Step ST1402,the CU duplicates a packet of the downlink user data, detects aredundant packet of the uplink user data, and removes the redundantpacket. In Steps ST1403 and ST1404, the CU transmits and receives theduplicated user data to and from the DU #1 and the DU #2, respectively.In Steps ST1405 and ST1406, the UE transmits and receives the duplicateduser data to and from the DU #1 and the DU #2, respectively. In StepST1407, the UE duplicates a packet of the uplink user data, detects aredundant packet of the downlink user data, and removes the redundantpacket.

Steps ST1408 to ST1413 of FIG. 18 indicate signaling for stopping thepacket duplication.

In Steps ST1408 and ST1409 of FIG. 18, the CU notifies the UE of thesignaling for the RRC connection reconfiguration. The CU transmits thenotification to the DU #1 in Step ST1408, and the DU #1 transmits thenotification to the UE in Step ST1409. The notification includesinformation indicating release of communication with the DU #2, andinformation indicating stop of packet duplication. The notification mayinclude information on the RRC parameter for the DU #1-correspondingentity. The UE sets the RRC parameter for the stop of packet duplicationand the release of communication with the DU #2, using the informationreceived in Step ST1409.

In Steps ST1408 and ST1409 of FIG. 18, the notification from the CU tothe UE may include information instructing change in the RRC parameterfor communication with the DU #1. The UE may change the setting of theRRC parameter for communication with the DU #1, using the informationreceived in Step ST1409. Consequently, the CU can flexibly set thecommunication with the DU #1 to correspond to the release of thecommunication with the DU #2.

In Steps ST1410 and ST1411 of FIG. 18, the UE transmits an RRCconnection reconfiguration complete notification to the CU. The UEtransmits the notification to the DU #1 in Step ST1410, and the DU #1transmits the notification to the CU in Step ST1411.

In Step ST1412 of FIG. 18, the CU notifies the DU #2 of an instructionfor stopping communication. The instruction includes informationindicating termination of use of the DU #2 in the communication with theUE.

In Step ST1413 of FIG. 18, the DU #2 notifies the CU of a response tothe instruction for stopping communication. The response includesinformation indicating completion of processing for stoppingcommunication in the DU #2.

In Steps ST1414 to ST1416 of FIG. 18, the CU transmits and receives theuser data to and from the UE through the DU #1. In Step ST1414, thehigh-level network device transmits and receives the user data to andfrom the CU. In Step ST1415, the CU transmits and receives the user datato and from the DU #1. In Step ST1416, the UE transmits and receives theuser data to and from the DU #1.

The stop of the packet duplication indicated in Steps ST1414 to ST1416of FIG. 18 may be performed before and after the response to theinstruction for stopping communication in Step ST1413. The stop of thepacket duplication before the response to the instruction for stoppingcommunication enables earlier release of unnecessary communicationresources and efficient communication. The stop of the packetduplication after the response to the instruction for stoppingcommunication can enhance the reliability of the user data communicationbefore and after the signaling for stopping the packet duplication.

The CU may use a combination of the instruction for startingcommunication and the instruction for stopping communication. Thiscombination may be used for switching between the use DUs. The CU mayintegrate notifications of the RRC connection reconfiguration into oneand give the one notification to the UE. This allows smooth switchingbetween the use DUs and also reduction in the amount of signaling.

The CU may add or delete the use DUs one by one. In other words, the CUmay notify each of the DUs of an instruction for starting communicationor an instruction for stopping communication. This facilitatesrestoration from a failure in a sequence for adding or deleting the useDUs.

Alternatively, the CU may collectively add or delete the use DUs. The CUmay integrate notifications of the RRC connection reconfiguration intoone and give the one notification to the UE. Consequently, the amount ofsignaling can be reduced.

The CU and the UE may communicate through a plurality of DUs incommunication in the C-Plane, similarly as in the U-Plane. The packetduplication may be applied to the communication through a plurality ofDUs, similarly as in the U-Plane. Consequently, reliability of thesignaling between the CU and the UE can be enhanced. The signaling maybe the NAS signaling, the RRC signaling, or the PDCP control PDU.Consequently, reliability of the signaling in each layer can beenhanced.

FIGS. 19 and 20 illustrate a sequence diagram of switching between theuse DUs and communication using a plurality of DUs in the C-Plane. FIGS.19 and 20 are connected across a location of a border BL1920. FIGS. 19and 20 illustrate an example of switching the use DUs from the DU #1 andthe DU #2 to the DU #1 and the DU #3. In FIGS. 19 and 20, the same stepnumbers are assigned to the same Steps as those in FIG. 18, and thecommon description thereof is omitted.

Steps ST1500 to ST1505 in FIG. 19 indicate a sequence on the signalingfor the RRC connection reconfiguration to be notified from the CU to theUE, specifically, a sequence on duplication and notification of a packetfor the signaling. In Step ST1500, the CU duplicates a packet forsignaling the RRC connection reconfiguration which has been generated byits own CU. The CU may duplicate the packet in the PDCP layer. Thesignaling for the RRC connection reconfiguration may include informationon change in the use DUs (switching to using the DU #1 and the DU #3 inthe example of FIGS. 19 and 20) and information indicating validation ofpacket duplication. In Steps ST1501 and ST1502, the CU forwards theduplicated signalings to the DU #1 and the DU #2, respectively. In StepsST1503 and ST1504, the DU #1 and the DU #2, respectively, notify thesignalings to the UE. In Step ST1505, the UE detects redundancy in thereceived signalings. Further, the UE removes the redundant signaling.The UE may detect and remove the redundant signaling in the PDCP layer.The UE changes the RRC parameter and the use DU, using the remainingsignaling.

Steps ST1506 to ST1511 in FIG. 19 indicate a sequence on the signalingfor completion of the RRC connection reconfiguration to be notified fromthe UE to the CU, specifically, a sequence on duplication andnotification of a packet for the signaling. In Step ST1506, the UEduplicates a packet for the signaling for completion of the RRCconnection reconfiguration which has been generated by its own CU. TheUE may duplicate the packet in the PDCP layer. In Steps ST1507 andST1508, the UE transmits the duplicated signalings to the DU #1 and theDU #2, respectively. In Steps ST1509 and ST1510, the DU #1 and the DU#2, respectively, forward the signalings to the CU. In Step ST1511, theCU detects redundancy in the received signalings. Further, the CUremoves the redundant signaling. The CU may detect and remove theredundant signaling in the PDCP layer. The CU recognizes that the UE hascompleted the RRC connection reconfiguration, using the remainingsignaling.

In Step ST1515 of FIG. 20, the CU notifies the DU #2 of an instructionfor stopping communication. The notification includes informationindicating termination of use of the DU #2 in the communication with theUE.

In Step ST1516 of FIG. 20, the DU #2 notifies the CU of a response tothe instruction for stopping communication. The response includesinformation indicating completion of processing for stoppingcommunication in the DU #2.

In Step ST1517 of FIG. 20, the CU notifies the DU #3 of an instructionfor starting communication. Step ST1517 may be identical to Step ST1007in FIG. 10. The DU #3 sets the RRC parameter for transmitting andreceiving data to and from the UE, using the information received inStep ST1517.

In Step ST1518 of FIG. 20, the DU #3 notifies the CU of a response tothe instruction for starting communication. The response may includeinformation indicating completion of the setting in the DU #3.

In Steps ST1520 and ST1521 of FIG. 20, a random access procedure isperformed for communication between the UE and the CU through the DU #3.Signaling is performed between the DU #3 and the CU in Step ST1520, anda radio signal is transmitted and received between the UE and the DU #3in ST1521. Steps ST1520 and ST1521 may be identical to Steps ST1009 andST1010 in FIG. 10, respectively.

In Steps ST1520 and ST1521 of FIG. 20, signaling necessary for switchingthe use DUs from the DU #1 and the DU #2 to the DU #1 and the DU #3 iscompleted.

Steps ST1525 to ST1531 in FIG. 20 indicate transmission and reception ofthe user data through packet duplication using the DU #1 and the DU #3.

In Step ST1525 of FIG. 20, the high-level network device transmits andreceives the user data to and from the CU. In Step ST1526, the CUduplicates a packet of the downlink user data, detects a redundantpacket of the uplink user data, and removes the redundant packet. InSteps ST1527 and ST1528, the CU transmits and receives the duplicateduser data to and from the DU #1 and the DU #3, respectively. In StepsST1529 and ST1530, the UE transmits and receives the duplicated userdata to and from the DU #1 and the DU #3, respectively. In Step ST1531,the UE duplicates a packet of the uplink user data, detects a redundantpacket of the downlink user data, and removes the redundant packet.

In FIG. 20, the sequence for the instruction for starting communicationwith the DU #3 in Step ST1517 and the response to the instruction forstarting communication in Step ST1518 may be performed before theinstruction for stopping communication is issued to the DU #2 in StepST1515. Issuance of the instruction for starting communication beforethe instruction for stopping communication enables the UE to expeditethe random access procedure through the DU #3.

In FIG. 20, the sequence for the instruction for stopping communicationwith the DU #2 in Step ST1515 and the response to the instruction forstopping communication in Step ST1516 and a sequence for the instructionfor starting communication with the DU #3 in Step ST1517 and theresponse to the instruction for starting communication in Step ST1518may be performed before a sequence for the RRC connectionreconfiguration in Steps ST1500 to ST1505. This can prevent re-executionof the RRC connection reconfiguration caused by failure of theinstruction for stopping or starting communication.

The UE may notify the CU of the failure of the RRC connectionreconfiguration. The notification may include a reason for the failure.The reason for the failure may be, for example, a shortage of theresources in the UE, disabled communication with the DU, or anotherreason. The CU may proceed with the processing using the reason for thefailure. The processing may be, for example, the use of another DU.Consequently, the CU and the UE can prevent the abort of the sequencedue to the failure of the RRC connection reconfiguration, and therelevant termination of operations.

The UE and the CU may continue communication using a parameter usedbefore the notification of the RRC connection reconfiguration. The UEand the CU may continue communication when the UE notifies the CU of afailure of the RRC connection reconfiguration. The UE may maintain anRRC_CONNECTED state in the continuation of the communication. The UEneed not notify the CU of a request for the RRC connectionre-establishment. Consequently, the communication between the UE and theCU can be maintained.

The notification of the RRC connection reconfiguration from the CU tothe UE may include an identifier specifying an operation of the UE uponfailure of the RRC connection reconfiguration. The identifier mayinclude information on whether the RRC_CONNECTED state is maintained orinformation indicating whether the UE needs to notify the CU of arequest for the RRC connection re-establishment. Consequently, the UEcan easily determine whether to continue communication upon failure ofthe RRC connection reconfiguration. Alternatively, the UE may determinean operation of its own UE, using an identifier indicating the presenceor absence of packet duplication. This enables reduction in the numberof bits for the signaling for the RRC connection reconfiguration.Moreover, the UE can easily determine whether to continue communicationupon failure of the RRC connection reconfiguration.

The notification of the failure of the RRC connection reconfigurationand the RRC connection reconfiguration complete notification, which aregiven from the UE to the CU, may be integrated into one notification.The integrated notification may have an identifier indicating completionor failure of the RRC connection reconfiguration. Alternatively, anidentifier indicating a reason for a failure of the RRC connectionreconfiguration may include indication of completion of the RRCconnection reconfiguration. For example, 0 that is a value indicating areason for the failure may be allocated to the completion of the RRCconnection reconfiguration. Since this reduces the types of signaling,the processing in the UE and the CU is facilitated.

FIGS. 21 and 22 illustrate a sequence diagram of operations upon failureof the RRC connection reconfiguration. FIGS. 21 and 22 are connectedacross a location of a border BL2122. FIGS. 21 and 22 illustrate anexample where an RRC connection reconfiguration for communicationbetween the CU and the UE using the DU #2 fails and an RRC connectionreconfiguration for communication between the CU and the UE using the DU#3 is completed. In FIGS. 21 and 22, the same step numbers are assignedto the same Steps as those in FIGS. 10 and 11 and FIGS. 19 and 20, andthe common description thereof is omitted.

In Steps ST1601 and ST1602 of FIG. 21, the UE notifies the CU of thefailure of the RRC connection reconfiguration. The notification mayinclude a reason for the failure. In the example of FIG. 21, thenotification includes unavailability of the DU #2 by the UE as a reasonfor the failure. In the example of FIG. 21, the CU may include, in aninstruction for the RRC connection reconfiguration, an instruction forusing a DU other than the DU #2 upon failure. Alternatively, the CU mayinclude, in the instruction for the RRC connection reconfiguration, aninstruction for reusing the DU #2 after a lapse of a certain time uponfailure. Consequently, the CU can flexibly select the DU even uponfailure of the RRC connection reconfiguration.

In Steps ST1610 and ST1611 of FIG. 21, the CU notifies the UE of thesignaling for the RRC connection reconfiguration. The notification isthe same as that in Steps ST1003 and ST1004 of FIG. 10 except for thedetails indicating that the use DU is the DU #3.

In Step ST1614 of FIG. 22, the CU notifies the DU #3 of an instructionfor starting communication. Step ST1614 may be identical to Step ST1007in FIG. 10. The DU #3 sets the RRC parameter for transmitting andreceiving data to and from the UE, using the information received inStep ST1614.

In Step ST1615 of FIG. 22, the DU #3 notifies the CU of a response tothe instruction for starting communication. Step ST1615 may be identicalto Step ST1008 in FIG. 10.

In Steps ST1616 and ST1617 of FIG. 22, a random access procedure isperformed for communication between the UE and the CU through the DU #3.Signaling is performed between the DU #3 and the CU in Step ST1616, anda radio signal is transmitted and received between the UE and the DU #3in ST1617. Steps ST1616 and ST1617 may be identical to Steps ST1009 andST1010 in FIG. 10, respectively.

In ST1617 of FIG. 22, signaling necessary for starting transmission andreception of the duplicated packets using the DU #1 and the DU #3 iscompleted.

The DU may transmit, to the CU, a notification of a failure in startingcommunication. The notification may include a reason for the failure.The reason for the failure may be, for example, a reason described in9.2.6 of Non-Patent Document 15 (3GPP TS 36.423 v14.2.0) or anotherreason. Since the reason for the failure for the DU to startcommunication can be notified similarly as the reason for the failure inthe conventional Xn interface, the complexity in designing the CU can beavoided. The CU may proceed with the processing using the reason for thefailure. The processing may be, for example, the use of another DU.Consequently, the CU and the UE can prevent the abort of the sequencedue to a failure of instructing the DU to start communication, and therelevant termination of operations.

A notification of a failure of an instruction for starting communicationand a notification of a response to the instruction for startingcommunication, which are given from the DU to the CU, may be integratedinto one notification. The integrated notification may have anidentifier indicating completion or failure of the instruction forstarting communication. Alternatively, an identifier indicating a reasonfor the failure of starting communication may include indication of aresponse to the instruction for starting communication. For example, 0that is a value indicating the reason for the failure may be allocatedto the response to the instruction for starting communication. Sincethis reduces the types of signaling, the processing in the DU and the CUis facilitated.

FIGS. 23 and 24 illustrate a sequence diagram of operations upon failureof the instruction for starting communication. FIGS. 23 and 24 areconnected across a location of a border BL2324. FIGS. 23 and 24illustrate an example where an RRC connection reconfiguration forcommunication between the CU and the UE using the DU #2 fails and an RRCconnection reconfiguration for communication between the CU and the UEusing the DU #3 is completed. In FIGS. 23 and 24, the same step numbersare assigned to the same Steps as those in FIGS. 10, 11, and 19 to 22,and the common description thereof is omitted.

In Step ST1701 of FIG. 23, the CU notifies the DU #2 of an instructionfor starting communication. Step ST1701 may be identical to Step ST1007in FIG. 10. The DU #2 sets the RRC parameter for transmitting andreceiving data to and from the UE, using the information received inStep ST1701.

In Step ST1702 of FIG. 23, the DU #2 notifies the CU of a failure ofstarting communication. The notification may include a reason for thefailure. In the example of FIG. 23, the DU #2 notifies the CU of ahardware fault as a reason for the failure. The CU may select another DUas the use DU, using the information received in Step ST1702.

The CU can maintain communication with the UE by selecting another DU asthe use DU, even when the CU fails to instruct the DU to startcommunication according to the sequence illustrated in FIGS. 23 and 24.

As an alternative example of an operation upon failure of instructing tostart communication, the CU may notify the UE of an instruction forstarting communication before the CU performs the RRC connectionreconfiguration for the UE. In FIGS. 23 and 24, for example, StepsST1701 and ST1614 indicating the time to start communication may beperformed before Steps ST1003, ST1004, ST1610, and ST1611 indicating theRRC connection reconfiguration. Since this eliminates the need forre-execution of the RRC connection reconfiguration caused by failure ofstarting communication in the DU, the amount of signaling can bereduced. In the example of FIGS. 23 and 24, Steps ST1003 and ST1004indicating the RRC connection reconfiguration and Steps ST1005 andST1006 indicating the completion of the RRC connection reconfigurationare unnecessary.

The DU may notify the CU of a failure of stopping communication. The DUmay give the notification after the CU instructs the DU to stopcommunication. The notification of the failure may be given in the samemanner as the notification of the failure in starting communication.This can produce the same advantages as those of the notification of thefailure in starting communication.

The CU may stop the user data communication with the UE through the DU.The CU may stop the user data communication with the DU that hasnotified the CU of a failure of stopping communication. This can preventunnecessary continuation of communication due to the failure of stoppingthe communication.

The CU may continue the user data communication with the UE through theDU. The CU may continue the user data communication with the DU that hasnotified the CU of a failure of stopping communication. This canmaintain the reliability of communication even upon failure of stoppingthe communication.

The DU may notify the CU of a failure of changing communication. The DUmay give the notification after the CU instructs the DU to changecommunication. The notification of the failure may be given in the samemanner as the notification of the failure in starting communication.This can produce the same advantages as those of the notification of thefailure in starting communication.

The CU may continue the user data communication with the UE through theDU. The CU may continue the user data communication with the DU that hasnotified the CU of a failure of changing communication. The CU maycontinue the user data communication using a parameter to be changed bythe instruction for changing communication. This can maintain thereliability of communication even upon failure of changing thecommunication.

The CU may determine a failure of an instruction for startingcommunication, due to no response from the DU that the CU has instructedto start communication for a predefined duration. The predefinedduration may be defined in advance in a standard, determined by the CU,or determined by the high-level network device and notified from thehigh-level network device to the CU. This can prevent termination ofoperations of the CU due to an undelivered response to the instructionfor starting communication from the DU.

As previously described, the CU may determine a failure of aninstruction for stopping communication due to no response from the DUthat the CU has instructed to stop communication for a predefinedduration. The CU may determine a failure of an instruction for changingcommunication due to no response from the DU that the CU has instructedto change communication for a predefined duration. This can preventtermination of operations of the CU due to an undelivered response tothe instruction for stopping or changing communication from the DU.

FIGS. 25 and 26 illustrate a sequence diagram of operations when aresponse to starting communication is undelivered from the DU to the CU.FIGS. 25 and 26 are connected across a location of a border BL2526.FIGS. 25 and 26 illustrate an example where there is no response fromthe DU #2 to the CU for a certain duration after the CU notifies the DU#2 of an instruction for starting communication. In FIGS. 25 and 26, thesame step numbers are assigned to the same Steps as those in FIGS. 23and 24, and the common description thereof is omitted.

In Step ST1801 of FIG. 25, the CU waits a certain duration for aresponse from the DU #2, after the CU notifies the DU #2 of aninstruction for starting communication in Step ST1701. If there is noresponse from the DU #2 for a certain duration in Step ST1801, the CUdetermines a failure in the instruction for starting communication.

The sequence illustrated in FIGS. 25 and 26 can prevent termination ofthe operations of the CU due to a wait for the response from the DU #2.For example, the CU can retransmit an instruction for startingcommunication to another DU.

The CU may transmit and receive, to and from the DU, data for checking anormal operation of a corresponding entity. The CU may regularlytransmit and receive the data to and from the DU. The CU and the DU maycheck a normal operation of a corresponding entity, using the data. TheCU may exclude, from the use DUs, a DU that does not transmit the datafor a certain duration. Alternatively, the CU may exclude, from thecandidate DUs, the DU that does not transmit the data for a certainduration. For example, the CU may exclude the DU that does not transmitthe data for a certain duration, from the targets to which aninstruction for starting, stopping, or changing communication isnotified. Consequently, the CU need not instruct the DU to start, stop,or change communication. This can prevent occurrence of a failuresequence for the DU and delete unnecessary signaling from the CU to theDU.

The data for checking a normal operation of a corresponding entity mayinclude only identifiers of the CU and the DU. Consequently, the amountof signaling for checking a normal operation of a corresponding entitycan be reduced.

The CU may notify the UE of the RRC connection reconfiguration. The RRCsignaling may be used for the notification. The parameter described in6.2.2 of 3GPP TS 36.311 v14.2.1 (Non-Patent Document 16) may be used forthe notification. Consequently, the complexity in designing the CU andthe UE on the notification of the RRC connection reconfiguration can beavoided.

The notification may include information of (1) to (8) below:

(1) an identifier indicating a bearer, for example, a bearer ID;

(2) information on packet duplication, for example, an identifierindicating the presence or absence of packet duplication;

(3) information on the DUs to be added;

(4) information on the DUs to be released;

(5) information on the DUs whose setting is to be changed;

(6) the number of the use DUs;

(7) an identifier indicating no re-establishment of an entity in eachlayer; and

(8) combinations of (1) and (7) above.

In (1), for example, a bearer ID (DRB-ID) may be used. Consequently, apacket in the U-Plane can be duplicated. Furthermore, an identifier (forexample, SRB-ID) of a Signaling Radio Bearer (abbreviated as SRB) may beused. The SRB-ID may be, for example, SRB0, SRB1, or SRB2. Consequently,a packet in the C-Plane can be duplicated.

In (2), the notification may include, for example, an identifierindicating the presence or absence of packet duplication. Consequently,the UE can easily identify the presence or absence of packetduplication. The notification may include information on a ratio oftransmitting duplicated packets to each DU. This enables flexible changein the ratio of duplicated packets in both the uplink communication andthe downlink communication. Furthermore, the notification may includeinformation on a priority of the PDCP-PDU received from each DU.Consequently, the amount of processing of detecting redundant PDCP-PDUsin the UE can be reduced.

In (3), the notification may include, for example, the number of the DUsto be added. Consequently, the UE can easily understand, via the RRCsignaling, a setting parameter for the DUs to be added, etc. Thenotification may include identifiers of the DUs to be added. Theidentifiers of the DUs may be the DU-IDs or cell IDs. Consequently, theUE can easily identify the corresponding DUs. The notification mayinclude the RRC parameters to correspond to the DUs. The RRC parametersmay include parameters on the New AS layer, the PDCP layer, the RLClayer, the MAC layer, and the PHY layer. Consequently, the communicationbetween the UE and the DU can be established.

In (4), the notification may include, for example, the number of the DUsto be released. Consequently, the UE can easily understand, via the RRCsignaling, a parameter for the DUs to be released, etc. The notificationmay also include IDs of the DUs to be released. The identifiers of theDUs may be the identifiers of the DUs in (3). This can produce the sameadvantages as those in (3). The notification may also include anidentifier indicating whether to release a split bearer between the DUsfor the DUs. When the split bearer between the DUs is used incommunication using a plurality of the DUs, the amount of processing ofreconfiguring the DUs once released can be reduced.

In (5), the notification may include, for example, the number of the DUsto be changed. Consequently, the UE can easily understand, via the RRCsignaling, a setting parameter for the DUs to be changed, etc. Thenotification may also include identifiers of the DUs to be changed. Theidentifiers of the DUs may be the identifiers of the DUs in (3). Thiscan produce the same advantages as those in (3). The notification mayalso include the RRC parameters to correspond to the DUs. The RRCparameters may be the RRC parameters in (3). This can produce the sameadvantages as those in (3).

In (6), notifying, from the CU to the UE, the number of DUs after changeenables the UE to easily understand the number of the use DUs after theRRC connection reconfiguration.

The layer to be instructed in (7) may be the New AS layer, the PDCPlayer, the RLC layer, the MAC layer, or the PHY layer. Prevention ofbuffer clearance caused by re-establishment of an entity in the layercan lead to prevention of data loss. Moreover, prevention ofretransmission from an upper layer in the UE can lead to reduction inthe latency of data transmission.

In (7), the entity in each layer may be re-established. This enablesreduction in the amount of processing of the UE for changing the RRCparameter.

The UE may notify the CU of completion of the RRC connectionreconfiguration. The RRC signaling may be used for the notification. TheRRC connection reconfiguration complete notification described in 6.2.2of 3GPP TS 36.311 v14.2.1 (Non-Patent Document 16) may be used for thenotification. Consequently, the complexity in designing the CU and theUE on the RRC connection reconfiguration complete notification can beavoided.

Alternatively, the PDCP sequence number that the UE has received may beincluded in the RRC connection reconfiguration complete notification.This can prevent the data loss in the RRC connection reconfiguration.The UE may notify the PDCP sequence number independently of the RRCconnection reconfiguration complete notification. Consequently, thecomplexity of design on the RRC connection reconfiguration completenotification in the CU and the UE can be avoided.

The RRC connection reconfiguration complete notification according tothe first embodiment may include a reason for a failure. The reason forthe failure may be the aforementioned reason for the failure included inthe notification of the failure of the RRC connection reconfiguration tobe transmitted from the UE to the CU. An identifier indicating thereason for the failure may include indication of completion of the RRCconnection reconfiguration. For example, 0 that is a value indicatingthe reason for the failure may be allocated to the completion of the RRCconnection reconfiguration. Since this reduces the types of signaling,the processing in the UE and the CU is facilitated.

The CU may notify the DU of an instruction for starting communication.The instruction may be issued through the interface between the CU andthe DU, for example, using signaling on the Fs interface. A parameterdescribed in “SeNB Addition Request” in 9.1.3.1 of 3GPP TS 36.423v14.2.0 (Non-Patent Document 15) may be used for the instruction. Aparameter to be applied to SCG-ConfigInfo described in 10.2.2 of 3GPP TS36.311 v14.2.1 (Non-Patent Document 16) may be used for the instruction.Consequently, the complexity in designing the CU and the DU on theinstruction for starting communication can be avoided.

The instruction may include information of (1) to (7) below:

(1) an identifier indicating a bearer, for example, a bearer ID;

(2) information on packet duplication, for example, an identifierindicating the presence or absence of packet duplication;

(3) an identifier of the DU, for example, a DU-ID;

(4) an identifier of the UE, for example, a UE-ID;

(5) the RRC parameters;

(6) an identifier indicating no re-establishment of an entity in eachlayer; and

(7) combinations of (1) and (6) above.

In (1), the same information as that of (1) in the notification of theRRC connection reconfiguration from the CU to the UE may be used. Thisproduces the same advantages as those in (1) in the notification of theRRC connection reconfiguration.

With application of (2), the CU and the DU can optimize the processingfor packet duplication, using the presence or absence of packetduplication.

The application of (3) can prevent a malfunction caused by theprocessing of another DU by wrongly receiving the instruction.

With application of (4), the DU can identify the UE to be acorresponding entity in the communication which is indicated by theinstruction.

The RRC parameters in (5) may include a parameter on the RLC layer, theMAC layer, or the PHY layer. Consequently, the communication between theUE and the DU can be established.

The layer to be instructed in (6) may be the RLC layer, the MAC layer,or the PHY layer. Prevention of the buffer clearance caused byre-establishment of an entity in the layer can prevent data loss.Moreover, prevention of retransmission from the PDCP layers in the CUand the UE can lead to reduction in the latency of data transmission.

In (6), the entity in each layer may be re-established. This enablesreduction in the amount of processing of the DU for changing the RRCparameter.

The DU may notify the CU of a response to an instruction for startingcommunication. Consequently, the CU can understand a state of the DU,for example, the presence or absence of a fault in the DU.

The response may include information of (1) to (4) below:

(1) an identifier indicating a bearer, for example, a bearer ID;

(2) an identifier of the DU, for example, a DU-ID;

(3) an identifier indicating completion or failure of the processinginvolved in the instruction for starting communication; and

(4) combinations of (1) and (3) above.

In (1), for example, when the CU notifies the DU of an instruction forstarting to communicate with a plurality of bearers, the CU can easilyidentify a bearer to be set by the DU.

In (2), for example, when the CU notifies a plurality of DUs ofinstructions for starting communication, the CU can easily identify a DUthat has transmitted the response.

In (3), an identifier indicating a reason for a failure may be included.A value indicating success in the processing involved in the instructionfor starting communication may be added to the identifier. The value maybe, for example, 0. This allows the integration of the response to theinstruction for starting communication with the notification of thefailure in starting communication, and reduction in the types ofsignaling.

The interface between the CU and the DU, for example, the Fs interfacemay be used for communication between the CU and the DU. The Fsinterface may be 8-bit aligned. Consequently, the amount of padding canbe reduced.

Alternatively, the Fs interface may be 64-bit aligned. This facilitatesalignment of data in communication using 64b/66b.

The Fs interface may be the one to which the methods defined in the CPRI(see Non-Patent Document 17) interface are applied as the interfacebetween the CU and the DU. This enables the interface to be sharedbetween an option in which the CU-DU split is performed in a lower layerand an option in which the CU-DU split is performed in a higher layer(for example, Option 2).

The control information may be located in a control block, and data maybe located in a data block. This can increase efficiency in thecommunication through the Fs interface.

The Fs interface may have a function of avoiding a collision. This canensure the low latency and the high reliability, and aggregate linesbetween the CU and a plurality of DUs.

The Fs interface may be the one to which the methods defined in the S1interface are applied as the interface between the CU and the DU.Consequently, the complexity in designing the Fs interface can beavoided.

The control information in the Fs interface may be shared with the RRCsignaling. Since the CU and the DU can easily convert the RRC signalingand the control information in the Fs interface, the processing time canbe reduced.

The DU may use the RRC signaling from the CU to the UE as the settinginformation for its own DU. Consequently, the signaling from the CU tothe DU (for example, an instruction for starting communication) can beintegrated with the RRC signaling from the CU to the UE (for example, anotification of the RRC connection reconfiguration). Consequently, theamount of signaling can be reduced.

The format of the control information in the Fs interface may be basedon, for example, the ASN.1. This can easily convert the controlinformation in the Fs interface into the RRC signaling and vice versa.

The technology for transmitting and receiving duplicated packets using aplurality of DUs, which is described in the first embodiment, may beapplied to the mobility between the DUs. In the mobility between theDUs, the addition and the release of the use DUs may be combined. Thiscan ensure the reliability before and after the mobility between theDUs.

When the technology for transmitting and receiving duplicated packetsusing a plurality of DUs is applied to the mobility between the DUs, theCU may notify the UE of information indicating transmission of theC-Plane data through a target DU. The CU may give the notification usingthe RRC signaling from the CU to the UE, for example, the RRC connectionreconfiguration. The RRC connection may be reconfigured when the use DUsare added in the mobility between the DUs. Consequently, the UE cansmoothly receive the RRC signaling when the use DUs are released in themobility between the DUs.

Alternatively, the CU may notify the UE of information indicating thetransmission through one of a target DU and a source DU, instead of theinformation indicating the transmission through a target DU. The CU maytransmit, to the UE through one of a target DU and a source DU, the RRCsignaling when the use DUs are released in the mobility between the DUs.For example, the CU may compare the target DU with the source DU, anduse the DU with a superior channel state to the UE. The UE may receivethe RRC signaling from both of the source DU and the target DU. Forexample, since the CU can use the DU with a superior channel state tothe UE, the reliability in transmitting the C-Plane data can beenhanced.

Alternatively, the CU may notify the UE of information indicating thetransmission through both of the target DU and the source DU, instead ofthe information indicating the transmission through the target DU. TheCU may transmit, to the UE through both of the target DU and the sourceDU, the RRC signaling when the use DUs are released in the mobilitybetween the DUs. The CU and the UE may apply, to transmission andreception of the signaling, the technology for transmitting andreceiving duplicated packets in the C-Plane using a plurality of DUs.The UE may receive the RRC signaling from both of the source DU and thetarget DU. This can further enhance the reliability in transmitting andreceiving the C-Plane data in the mobility between the DUs.

FIGS. 27 and 28 illustrate a sequence for the mobility between the DUsusing the technology for transmitting and receiving duplicated packets.FIGS. 27 and 28 are connected across a location of a border BL2728.FIGS. 27 and 28 illustrate an example of switching the communicationbetween the CU and the UE from the communication using the DU #1 to thecommunication with application of duplicated packets using the DU #1 andthe DU #2, and then to the communication using the DU #2. In FIGS. 27and 28, the same step numbers are assigned to the same Steps as those inFIGS. 10 and 11 and FIGS. 14 and 15, and the common description thereofis omitted.

In Step ST1904 of FIG. 27, the UE measures signals to be transmittedfrom the DU #1 and the DU #2. In Steps ST1905 and ST1906, the UEnotifies the CU of the measurement result. The RRC signaling may be usedfor the notification. The UE transmits the measurement result to the DU#1 in Step ST1905, and the DU #1 transmits the measurement result to theCU in Step ST1906. In Step ST1907, the CU determines the presence orabsence of the mobility between the DUs, using the measurement result.In the example of FIG. 27, in Step ST1907, the CU determines to performpacket duplication using the DU #1 and the DU #2.

In the sequence of Steps ST1007, ST1008, ST1201 to ST1205, ST1009, andST1010 of FIG. 27, the DU #2 is added and the setting for the packetduplication is made.

In Steps ST1908 to ST1914 in FIG. 28, the user data is communicatedthrough packet duplication using the DU #1 and the DU #2. Steps ST1908to ST1914 may be identical to Steps ST1401 to ST1407 in FIG. 18,respectively.

In Steps ST1915 to ST1917 in FIG. 28, the same processing as StepsST1904 to ST1906, respectively, is performed. In Step ST1918, the CUdetermines the presence or absence of the mobility between the DUs,using the measurement result obtained in Step ST1917. In the example ofFIG. 28, the CU determines to perform communication using only the DU #2in Step ST1918.

In FIG. 28, the CU instructs the DU #1 to stop communication in StepsST1919, and the DU #1 responds to the instruction for stoppingcommunication to the CU in ST1920. Steps ST1919 and ST1920 may beidentical to Steps ST1412 and ST1413 in FIG. 18, respectively.

In Steps ST1921 to ST1924 of FIG. 28, the CU sets the RRC connectionreconfiguration to the UE, and the UE gives the RRC connectionreconfiguration complete notification to the CU. In the example of FIG.28, the DU #1 is released, and the setting for cancelling packetduplication is made. Step ST1921 indicates transmission of the RRCconnection reconfiguration from the CU to the DU #2. Step ST1922indicates transmission of the RRC connection reconfiguration from the DU#2 to the UE. Step ST1923 indicates transmission of the RRC connectionreconfiguration complete notification from the UE to the DU #2. StepST1924 indicates the RRC connection reconfiguration completenotification from the DU #2 to the CU.

In Step ST1925 of FIG. 28, the CU notifies the DU #1 of completion ofthe UE reconfiguration. The DU #1 may stop communication with the UEusing the notification.

In Steps ST1930 to ST1932 of FIG. 28, the user data is communicatedusing the DU #2. Step ST1930 indicates transmission of the user databetween the high-level network device and the CU. Step ST1931 indicatestransmission and reception of the user data between the CU and the DU#2. Step ST1932 indicates transmission and reception of the user databetween the DU #2 and the UE.

In FIGS. 27 and 28, the use DU in a notification of the measurementresult may be a source DU. Since this eliminates the need for aninstruction for changing the DU, the amount of signaling can be reduced.

The technology for transmitting and receiving duplicated packets using aplurality of DUs which is described in the first embodiment may beapplied to the mobility between the CUs. In the application to themobility between the CUs, for example, an identifier of the DU to beused in a target base station and an identifier indicating packetduplication should be included in a Handover Request ACK from the targetbase station to a source base station as described in Non-PatentDocument 1. The same should be included in a notification of the RRCconnection reconfiguration from the source base station to the UE. Thiscan ensure the reliability before and after the mobility between theCUs.

Since the first embodiment enables the packet communication using aplurality of DUs, the high reliability and the low latency in thecommunication between the CU and the UE can be ensured.

The first embodiment provides, for example, the following configuration.

Provided is a communication system including a communication terminaldevice, and a base station device configured to perform radiocommunication with the communication terminal device. Specifically, thebase station device includes: a plurality of distributed units (DUs)that transmit and receive radio signals; and a central unit (CU) thatcontrols the plurality of DUs. The CU duplicates a downlink packetaddressed to the communication terminal device, and forwards theduplicated downlink packet to each of at least two DUs among theplurality of DUs. Each of the at least two DUs transmits, to thecommunication terminal device by the radio signal, the downlink packetobtained from the CU. Upon redundant receipt of the downlink packets,the communication terminal device removes a redundant downlink packet inaccordance with a predefined downlink packet removal criterion.

Here, the communication terminal device may transmits, to each of two ormore of the plurality of DUs, a duplicated uplink packet of an uplinkpacket to be transmitted from the communication terminal device. In sucha case, upon redundant receipt of the uplink packets, the base stationdevice removes a redundant uplink packet in accordance with a predefineduplink packet removal criterion.

The configuration can be variously modified based on the disclosure andthe suggestion of the Description including the first embodiment. Theconfiguration and the modified configuration can solve the problems, andproduce the advantages.

First Modification of First Embodiment

Although the first embodiment describes the packet duplication in Option2 of the CU-DU split, the packet duplication may be applied to Option3-1 of the CU-DU split.

The CU duplicates a packet forwarded from a high-level network device.The CU may duplicate the packet in the PDCP layer. The CU forwards theduplicated packet to an RLC-H layer corresponding to each DU. Each ofthe RLC-H layers transmits the packet to the corresponding DU. Each ofthe DUs transmits the packet to the UE. The UE detects redundantpackets. The UE removes the redundant packets.

The operations of the CU, the DUs, and the UE may be performed in thedownlink communication.

The UE duplicates a packet, and transmits the duplicated packet to eachof the DUs through a lower-layer entity that corresponds to the DU. Eachof the DUs forwards the received packet to the CU. The CU forwards thepackets received from the DUs to the PDCP layer. The PDCP layer of theCU detects redundancy. The PDCP layer of the CU removes the redundantpackets. The CU forwards the packet that is not removed to thehigh-level network device.

The operations of the CU, the DUs, and the UE may be performed in theuplink communication.

FIG. 29 illustrates a configuration for duplicating a packet in the PDCPlayer in the downlink communication using a plurality of DUs, in Option3-1 of the CU-DU split. In FIG. 29, the same numbers are assigned to thesame blocks as those in FIG. 8, and the common description thereof isomitted.

In FIG. 29, a PDCP layer 809 forwards a PDCP-PDU #1 (PDCP-PDU 812) to anRLC-H layer 2001. The PDCP layer 809 also forwards a PDCP-PDU #2(PDCP-PDU 813) to an RLC-H layer 2002.

In FIG. 29, the RLC-H layer 2001 assigns an RLC header 2005 to thePDCP-PDU #1 to generate an RLC-PDU #1 (may be referred to as an RLC-PDU2008). The RLC-H layer 2001 forwards the RLC-PDU #1 to an RLC-L layer2012 of a DU #1 (DU 802) through the Fs interface 814. Similarly, theRLC-H layer 2002 assigns an RLC header 2006 to the PDCP-PDU #2 togenerate an RLC-PDU #2 (may be referred to as an RLC-PDU 2009). TheRLC-H layer 2002 forwards the RLC-PDU #2 to an RLC-L layer 2013 of a DU#2 (DU 803) through the Fs interface 815.

In FIG. 29, the DU #1 transmits the RLC-PDU #1 received in the RLC-Llayer 2012 to a DU #1-corresponding entity 2015 in the UE 804. The DU #2transmits the RLC-PDU #2 received in the RLC-L layer 2013 to a DU#2-corresponding entity 2016 in the UE 804. An RLC-L layer 2017 forwardsthe received RLC-PDU #1 to an RLC-H layer 2020. An RLC-L layer 2018forwards the received RLC-PDU #2 to an RLC-H layer 2021. The RLC-H layer2020 removes the RLC header 2005 from the RLC-PDU #1 to obtain thePDCP-PDU #1, and forwards the PDCP-PDU #1 to the PDCP layer 822.Similarly, the RLC-H layer 2021 removes the RLC header 2006 from theRLC-PDU #2 to obtain the PDCP-PDU #2, and forwards the PDCP-PDU #2 tothe PDCP layer 822.

The operations of the PDCP layer 822 for detecting redundant packets andremoving the redundant packets in FIG. 29 are the same as those in FIG.8.

The other detailed operations are the same as those in the firstembodiment. Thus, the description is omitted. In Option 3-1 of the CU-DUsplit, application of methods similar to those in the first embodimentproduces the same advantages as those in the first embodiment.

The first modification of the first embodiment even with application ofOption 3-1 of the CU-DU split can ensure the high reliability and thelow latency in the communication through packet duplication.

Second Embodiment

In NR, a proposal is made on packet duplication in a layer lower thanthe PDCP layer, that is, the RLC layer or the MAC layer to promptlyaddress changes in a channel state while ensuring the high reliabilityand the low latency (see Non-Patent Document 14 (3GPP R2-1701472)).

The first modification of the first embodiment discloses a method forcommunicating with the UE using a plurality of DUs being served by theCU through the packet duplication in the PDCP layer In Option 3-1 of theCU-DU split.

According to the first modification of the first embodiment, however,the RLC-H layers lower than the PDCP layer also exist in the CU. Thiscauses problems of increasing the buffer usage in the RLC-H layers andincreasing the amount of processing in the PDCP layer and the RLC-Hlayers.

The second embodiment discloses a method for solving such problems.

The CU duplicates a packet in the RLC-H layer. The RLC-H layer forwardsthe duplicated packet to the RLC-L layer of each DU. In the UE, theRLC-L layer corresponding to each DU receives the packet transmittedfrom the DU, and the RLC-H layer receives the packet from the RLC-Llayer corresponding to the DU. The RLC-H layer of the UE detectsredundant packets. The RLC-H layer of the UE removes the redundantpackets.

The operations of the CU, the DUs, and the UE may be performed in thedownlink communication.

The UE duplicates a packet in the RLC-H layer, and transmits theduplicated packet to each of the DUs through an RLC-L layer entity thatcorresponds to the DU. The RLC-L layer in each of the DUs forwards thereceived packet to the CU. The RLC-H layer of the CU detects redundantpackets received from the RLC-L layers of the DUs. The RLC-H layer ofthe CU removes the redundant packets. The RLC-H layer of the CU forwardsthe packet that is not removed to the high-level network device throughan upper layer.

The operations of the CU, the DUs, and the UE may be performed in theuplink communication.

The CU and/or the UE may give the duplicated packets the same RLCsequence number. Consequently, the UE and/or the CU as a receiver easilydetects redundant packets.

The CU and/or the UE may give the RLC sequence number in the same manneras giving the PDCP sequence number according to the first embodiment.Consequently, the UE and/or the CU as a receiver easily detectsredundant packets similarly as the first embodiment.

The second embodiment differs from the first embodiment and the firstmodification of the first embodiment in applying the RLC sequencenumber, whereas the PDCP sequence number is applied in the firstembodiment and the first modification of the first embodiment. Thesecond embodiment also differs from Non-Patent Document 14 in that theCU performs transmission and reception to and from the UE using aplurality of DUs being served thereby through packet duplication in theRLC-H layer.

FIG. 30 illustrates a configuration for duplicating a packet in theRLC-H layer in the downlink communication using a plurality of DUs, inOption 3-1 of the CU-DU split. In FIG. 30, the same numbers are assignedto the same blocks as those in FIG. 29, and the common descriptionthereof is omitted.

In FIG. 30, the PDCP layer 2104 assigns a PDCP header 2100 to thePDCP-SDU 808 to generate a PDCP-PDU 2101. The PDCP layer 2104 forwardsthe PDCP-PDU 2101 to an RLC-H layer 2105.

In FIG. 30, the RLC-H layer 2105 duplicates the PDCP-PDU 2101 into two,assigns an RLC header 2106 to one of the duplicated PDUs to generate anRLC-PDU #1 (may be referred to as an RLC-PDU 2108), and assigns an RLCheader 2107 to the other duplicated PDU to generate an RLC-PDU #2 (maybe referred to as an RLC-PDU 2109). Although the RLC headers 2106 and2107 include information on the same sequence number #m in FIG. 30, theymay include information on different sequence numbers. For example,assigning serial sequence numbers to the RLC headers 2106 and 2107facilitates design of the sequence number assigner in the RLC layer.

In FIG. 30, the RLC-H layer 2105 forwards the RLC-PDU #1 to the RLC-Llayer 2012 of the DU #1 through the Fs interface 814. The RLC-H layer2015 also forwards the RLC-PDU #2 to the RLC-L layer 2013 of the DU #2through the Fs interface 815.

The RLC-L layer 2017 of the UE 804 forwards the received RLC-PDU #1 toan RLC-H layer 2110. The RLC-L layer 2018 of the UE 804 forwards thereceived RLC-PDU #2 to the RLC-H layer 2110.

In FIG. 30, the RLC-H layer 2110 detects redundant packets. In theexample of FIG. 30, the RLC-H layer 2110 detects that the RLC-PDU #1 andthe RLC-PDU #2 are the same, and removes the RLC-PDU #2. The RLC-H layer2110 removes the RLC header 2106 from the RLC-PDU #1 to obtain thePDCP-PDU 2101, and forwards the PDCP-PDU 2101 to the PDCP layer 2111.Although the RLC-H layer 2110 removes the RLC-PDU #2 in the example ofFIG. 30, it may remove the RLC-PDU #1. In such a case, the RLC-H layer2110 removes the RLC header 2107 from the RLC-PDU #2 to obtain thePDCP-PDU 2101, and forwards the PDCP-PDU 2101 to the PDCP layer 2111.

In FIG. 30, the PDCP layer 2111 removes the PDCP header 2100 from thePDCP-PDU 2101, and forwards the obtained PDCP-SDU 808 to the New ASlayer 823.

Other detailed operations are identical to those in the firstembodiment. Thus, the description is omitted. In Option 3-1 of the CU-DUsplit, application of the methods similar to those in the firstembodiment produces the same advantages as those in the firstembodiment.

The second embodiment produces the same advantages as those according tothe first modification of the first embodiment. Since the buffer usagein the RLC-H layer can be reduced more than that according to the firstmodification of the first embodiment, the buffer capacity in the RLC-Hlayer can be reduced. Since the packet duplication and detection ofredundant packets are performed in a layer lower than those according tothe first embodiment and the first modification of the first embodiment,the packet processing in the CU and the UE is expedited.

The second embodiment provides, for example, the following configurationsimilarly as the first embodiment.

Provided is a communication system including a communication terminaldevice, and a base station device configured to perform radiocommunication with the communication terminal device. Specifically, thebase station device includes: a plurality of distributed units (DUs)that transmit and receive radio signals; and a central unit (CU) thatcontrols the plurality of DUs. The CU duplicates a downlink packetaddressed to the communication terminal device, and forwards theduplicated downlink packet to each of at least two DUs among theplurality of DUs. Each of the at least two DUs transmits, to thecommunication terminal device by the radio signal, the downlink packetobtained from the CU. Upon redundant receipt of the downlink packets,the communication terminal device removes a redundant downlink packet inaccordance with a predefined downlink packet removal criterion.

Here, the communication terminal device may transmit, to each of two ormore of the plurality of DUs, a duplicated uplink packet of an uplinkpacket to be transmitted from the communication terminal device. In sucha case, upon redundant receipt of the uplink packets, the base stationdevice removes a redundant uplink packet in accordance with a predefineduplink packet removal criterion.

The configuration can be variously modified based on the disclosure andthe suggestion of the Description including the second embodiment. Theconfiguration and the modified configuration can solve the problems, andproduce the advantages.

Third Embodiment

The first embodiment, the first modification of the first embodiment,and the second embodiment disclose a method for duplicating a packet andtransmitting and receiving the packets using a plurality of DUs toensure the low latency and the high reliability.

Even upon occurrence of the mobility between the DUs, application of thepacket duplication can ensure the reliability in transmitting andreceiving data before and after the mobility between the DUs.

The packet duplication, however, creates a problem of consuming a largeamount of radio resources or buffer in the DUs being served by the CU.

The third embodiment discloses a method for solving such a problem.

The DU notifies the CU of information on the PDCP sequence number. TheDU notifies the CU of the sequence number of the PDCP-PDU with HARQacknowledgement in its own DU, as the PDCP sequence number. The CUtransmits, to the DU, a PDCP-PDU that does not correspond to the PDCPsequence number.

The operation may be performed upon occurrence of the mobility betweenthe DUs. For example, the DU that notifies the CU of information on thePDCP sequence number may be a source DU. The DU to which the CUtransmits the PDCP-PDU that does not correspond to the PDCP sequencenumber may be a target DU. This enables the CU to retransmit, to thetarget DU, the PDCP-PDU having no acknowledgement from the source DU tothe UE, upon occurrence of the mobility between the DUs. This canprevent the PDCP-PDU loss in the mobility between the DUs. Moreover, thePDCP-PDU can be promptly retransmitted.

The operation may be performed upon no mobility between the DUs. Forexample, the DU that notifies the CU of the information on the PDCPsequence number may be the same as the DU to which the CU transmits thePDCP-PDU that does not correspond to the PDCP sequence number. As anexample application upon no mobility between the DUs, upon excess of themaximum HARQ retransmission times, the DU may notify the CU of thePDCP-PDU PDCP sequence number included in the transport block datasubject to the excess HARQ retransmissions. This enables, for example,the CU to promptly retransmit, to the DU, the PDCP-PDU included in thetransport block data subject to the excess HARQ retransmissions.

The DU may notify the PDCP sequence number through the interface betweenthe CU and the DU, for example, the Fs interface. Similarly as the firstembodiment, the notification through the Fs interface may be the one towhich the methods defined in the S1 interface are applied as theinterface between the CU and the DU. Consequently, the complexity indesigning the Fs interface can be avoided. For example, transmission ofthe PDCP sequence number as the control information of the Fs interfaceenables a notification with a smaller overhead.

Non-Patent Document 18 (3GPP R2-1701461) proposes retransmission controlin the PDCP layer as the PDCP ARQ. The third embodiment differs fromNon-Patent Document 18 in eliminating the need for feedback of the PDCPsequence number from the receiver.

In the application of the third embodiment to the mobility between theDUs, conditions for the occurrence of the mobility between the DUs maybe similar to the conventional conditions for the occurrence of themobility between the base stations. For example, a difference insignal-to-noise ratio (abbreviated as SNR) between the DUs may be usedwith application of a condition described in Non-Patent Document 12,that is, a condition that a difference in SNR is higher than or equal toa certain threshold, or lower than or equal to the certain threshold.This can avoid the complexity in processing the mobility between theDUs.

The PDCP sequence number used in the third embodiment may be determinedusing HARQ-ACK from the UE. This enables the acknowledgement in the PDCPlayer even when the PDCP layer or the RLC layer uses RLC-UM that doesnot use feedback performed by the corresponding entity foracknowledgement.

The PDCP sequence number may be a PDCP sequence number of the PDCP-PDUthat has been transmitted the earliest among PDCP-PDUs having noacknowledgement in the HARQ. The CU may forward the PDCP-PDU with thePDCP sequence number to the target DU. Since this eliminates the needfor the CU to perform the increment processing on the PDCP sequencenumber, the amount of processing in the CU can be reduced.

Alternatively, the PDCP sequence number may be the last PDCP sequencenumber of the consecutive PDCP-PDUs with HARQ acknowledgement in the DU.Consequently, the amount of signaling from the DU to the CU can bereduced.

Alternatively, the source DU may notify the CU of, for example, adelivery state of PDCP-PDUs in bitmap format as information indicatingthe PDCP sequence number. For example, when PDCP-PDUs having noacknowledgement are non-consecutive, the CU can efficiently retransmitthe PDCP-PDUs having no acknowledgement to the target DU.

Alternatively, the information indicating the PDCP sequence number maybe, for example, combined information of a PDCP sequence number of aPDCP-PDU that has been transmitted last among PDCP-PDUs withacknowledgement, and a PDCP sequence number of a PDCP-PDU having noacknowledgement before the PDCP sequence number of the PDCP-PDU that hasbeen transmitted last. Consequently, the CU can efficiently retransmitPDCP-PDUs having no acknowledgement to the target DU while reducing theamount of signaling for notification from the source DU to the CU.

Alternatively, the information indicating the PDCP sequence number maybe, for example, combined information of a PDCP sequence number of aPDCP-PDU that has been transmitted the earliest among PDCP-PDUs havingno acknowledgement, and a PDCP sequence number of a PDCP-PDU withacknowledgement after the PDCP sequence number of the PDCP-PDU that hasbeen transmitted the earliest. This can produce the same advantages aspreviously described, particularly when the number of PDCP-PDUs withacknowledgement is less.

Alternatively, the information indicating the PDCP sequence number maybe of the sequence number of the PDCP-PDU including data that is yet tobe scheduled in the HARQ layer of the source DU. Consequently, the CUand the DU can respond earlier due to the mobility between the DUs.

The DU may associate, with information on the PDCP-SN, information onthe HARQ-ACK received from the UE. The DU should always associate themduring the continued communication between the DU and the UE. Thisenables earlier notification of the PDCP sequence number to the CU uponoccurrence of the mobility between the DUs.

An example method for associating the information on the HARQ-ACK withthe information on the PDCP-SN is hereinafter disclosed.

The RLC layer of the DU obtains the PDCP sequence number from thePDCP-PDU received from the PDCP layer of the CU. The RLC layer of the DUassociates the PDCP sequence number with the RLC sequence number of theRLC-PDU that its own RLC layer has transmitted to the MAC layer usingthe PDCP-PDU.

The MAC layer of the DU obtains the RLC sequence number from the RLC-PDUreceived from the RLC layer. The MAC layer of the DU associates the RLCsequence number with a HARQ process number used by its own MAC layer fortransmitting the transport block data to the UE using the RLC-PDU.

The MAC layer of the DU obtains the HARQ process number withacknowledgement, using the HARQ-ACK information from the UE. The MAClayer of the DU obtains the RLC sequence number with acknowledgementusing the HARQ process number and the information for associating theRLC sequence number with the HARQ process number. The MAC layer of theDU notifies the RLC layer of the DU of information on the RLC sequencenumber.

The RLC layer of the DU obtains the PDCP sequence number withacknowledgement, using the information on the RLC sequence numbernotified from the MAC layer of the DU, and information for associatingthe PDCP sequence number with the RLC sequence number.

The associating method eliminates the need for the feedback informationon the RLC layer and the PDCP layer from the UE, in acknowledgement ofthe PDCP-PDU. This can expedite the acknowledgement of the PDCP-PDU.

When the DU notifies the CU of the PDCP sequence number having noacknowledgement, the RLC layer of the DU may obtain the PDCP sequencenumber having no acknowledgement, using the PDCP sequence numberobtained by its own RLC layer from the PDCP-PDU and the PDCP sequencenumber with acknowledgement. The PDCP sequence number having noacknowledgement may be, for example, a PDCP sequence number obtained byexcluding the PDCP sequence number with acknowledgement from the PDCPsequence number obtained from the PDCP-PDU. This enables the DU topromptly notify the CU of information on the PDCP sequence number havingno acknowledgement.

In Non-Patent Document 19 (3GPP R2-1700177), HARQ-Nack from the UE isused for acknowledgement in the RLC layer. In contrast, the thirdembodiment differs from Non-Patent Document 19 in acknowledgement usingHARQ-ACK. The third embodiment also differs from Non-Patent Document 19in acknowledgement in the PDCP layer by associating the RLC sequencenumber with the PDCP sequence number.

Upon occurrence of the mobility between the DUs, transmission of thesource DU may be stopped after data stored in the buffer of the DU istransmitted to the UE. The data may be HARQ retransmission data.Alternatively, the data may include data that is not scheduled and isstored in the RLC buffer. This can enhance the reliability intransmission of the data stored in the buffer of the DU.

Alternatively, upon occurrence of the mobility between the DUs,transmission of the source DU may be stopped without transmitting, tothe UE, the data stored in the buffer of the DU. This can expeditecompletion of the mobility between the DUs.

FIG. 31 illustrates a configuration for PDCP acknowledgement usingHARQ-ACK. FIG. 31 illustrates an example of switching the DU to be usedby the CU and the UE from the DU #1 to the DU #2. In FIG. 31, the samenumbers are assigned to the same blocks as those in FIGS. 8 and 30, andthe common description thereof is omitted.

In FIG. 31, an RLC layer 2202 of the DU #1 (DU 802) obtains the PDCPsequence number from the PDCP-PDU obtained from a PDCP layer 2201 of theCU 801. The RLC layer 2202 forwards the RLC-PDU generated using thePDCP-PDU to a MAC layer 2203, and manages the RLC sequence number of theRLC-PDU and the PDCP sequence number in association with one another.

In FIG. 31, the MAC layer 2203 obtains the RLC sequence number from theRLC-PDU. The MAC layer 2203 transmits the transport block data generatedusing the RLC-PDU to the UE 804, and manages, in association with theRLC sequence number, the HARQ process number used for transmitting thetransport block data.

In FIG. 31, assume that a MAC layer 2210 of the UE 804 accuratelyreceives user data (the transport block generated using the RLC-PDU2215) from the MAC layer 2203 of the DU #1. The MAC layer 2210 notifiesthe MAC layer 2203 of the DU #1 of HARQ-ACK information 2205.

In FIG. 31, the MAC layer 2203 of the DU #1 obtains the HARQ processnumber with HARQ-ACK, from the HARQ-ACK information 2205. The MAC layer2203 finds the RLC sequence number with acknowledgement, using the HARQprocess number and the association between the HARQ process number andthe RLC sequence number. The MAC layer 2203 transmits the RLC sequencenumber to the RLC layer 2202 as an RLC sequence number notification2206.

In FIG. 31, the RLC layer 2202 obtains the PDCP sequence number withacknowledgement, using the RLC sequence number notification 2206 and theassociation between the RLC sequence number and the PDCP sequencenumber.

In FIG. 31, the RLC layer 2202 and the MAC layer 2203 may update the RLCsequence number with acknowledgement and the PDCP sequence number withacknowledgement, using the HARQ-ACK information from the MAC layer 2210of the UE 804 at any time. This enables the DU #1 to promptly notify theCU of the PDCP sequence number upon occurrence of the mobility betweenthe DUs. Alternatively, the update may be performed upon occurrence ofthe mobility between the DUs. This can reduce the amount of processingin the RLC layer 2202 and the MAC layer 2203.

In FIG. 31, assume the occurrence of the mobility between the DUs,specifically, from the DU #1 to the DU #2 when the PDCP sequence numberwith acknowledgement is n. Upon occurrence of the mobility between theDUs, specifically, from the DU #1 to the DU #2, the RLC layer 2202transmits the PDCP sequence number n with acknowledgement to the PDCPlayer 2201 of the CU 801 as a PDCP sequence number notification 2207.The Fs interface 814 is used for the transmission.

In FIG. 31, the PDCP layer 2201 transmits a PDCP-PDU 2211 of a PDCPsequence number (for example, n+1) having no acknowledgement to the DU#2 (DU 803), using the PDCP sequence number notification 2207. The Fsinterface 815 is used for the transmission.

The configuration illustrated in FIG. 31 enables the PDCP layer 2201 topromptly retransmit, to the UE 804 using the DU #2, the PDCP-PDU thatthe UE cannot accurately receive upon occurrence of the mobility fromthe DU #1 to the DU #2.

FIGS. 32 to 34 illustrate a sequence on the mobility for the PDCPacknowledgement using HARQ-ACK. FIGS. 32 to 34 are connected acrosslocations of borders BL3233 and BL3334. FIGS. 32 to 34 illustrate anexample of switching the DU to be used by the CU and the UE from the DU#1 to the DU #2. In FIGS. 32 to 34, the same step numbers are assignedto the same Steps as those in FIGS. 27 and 28, and the commondescription thereof is omitted.

In Step ST2301 of FIG. 32, the CU transmits, to the DU #1, the PDCP-PDUof user data for the UE. In Step ST2302, the RLC layer of the DU #1obtains the PDCP sequence number (PDCP-SN) from the PDCP-PDU of StepST2301. In Step ST2303, the DU #1 transmits the user data of Step ST2301to the UE.

In Step ST2304 of FIG. 32, the UE notifies the MAC layer of the DU #1 ofHARQ-ACK information. In Step ST2305, the MAC layer of the DU #1 updatesan acknowledgement HARQ-ID, using the HARQ-ACK information of StepST2304. In Step ST2306, the MAC layer of the DU #1 updates anacknowledgement RLC sequence number using the HARQ-ID, and notifies theRLC layer of the DU #1 of the RLC sequence number. In Step ST2307, theRLC layer of the DU #1 updates an acknowledgement PDCP sequence numberusing the RLC sequence number.

In Steps ST1904 to ST1906 and ST2315 of FIG. 32, the UE performsmeasurements on the DU #1 and the DU #2 and notifies the CU of themeasurement result, and the CU determines the mobility between the DUs.In the example of FIG. 32, the mobility from the DU #1 to the DU #2 isdetermined in Step ST2315.

In Step ST2320 of FIG. 33, the DU #1 notifies the CU of information onthe acknowledgement PDCP sequence number updated in Step ST2307. Thenotification of Step ST2320 may be given together with the response tothe instruction for stopping communication in Step ST1920. Consequently,the amount of signaling from the DU #1 to the CU can be reduced.

In Steps ST2325 and ST2326 of FIG. 33, the CU notifies the UE of the RRCconnection reconfiguration through the DU #1. In Steps ST2325 andST2326, the CU instructs the UE to switch from the DU #1 to the DU #2.In Steps ST2327 and ST2328, the UE notifies the CU of completion of theRRC connection reconfiguration through the DU #1.

In Steps ST1205, ST1925, ST1009, and ST1010 of FIG. 33, switching fromthe DU #1 to the DU #2 is completed.

In Step ST2341 of FIG. 34, the CU transmits, to the DU #2, the PDCP-PDUwith the PDCP sequence number which is yet to be acknowledged, using thePDCP sequence number information notification of Step ST2320. FIG. 34exemplifies that the PDCP-PDU with the PDCP sequence number which is yetto be acknowledged is a PDCP-PDU with a PDCP sequence number subsequentto the PDCP sequence number of Step ST2320.

In Steps ST2342 to 2347 of FIG. 34, the DU #2 performs the sameprocessing as Steps ST2302 to 2307, respectively.

In FIGS. 32 to 34, the PDCP sequence number information notification ofStep ST2320 may be given after Step ST1925, that is, after notificationof the UE reconfiguration completion from the CU to the DU #1.Consequently, the DU #1 can notify the CU of the PDCP sequence numberwith acknowledgement immediately before the DU is switched. Thus,redundant transmission of the PDCP-PDU can be reduced.

In an alternative example sequence on the mobility for the PDCPacknowledgement using HARQ-ACK, the UE may determine the mobility. Sincethis eliminates the need for notification of a measurement result, theamount of signaling through a radio interface can be reduced.

In FIG. 33, the DU #1 may transmit the data stored in the buffer of itsown DU to the CU after the notification of the UE reconfigurationcompletion in Step ST1925. The data may include the HARQ retransmissiondata, or the data stored in the RLC buffer, that is, data yet-to-be-HARQscheduled. This can enhance the reliability in transmission of data uponoccurrence of the mobility between the DUs.

Alternatively, in FIG. 33, the DU #1 may discard the data stored in thebuffer of its own DU after the notification of the UE reconfigurationcompletion in Step ST1925. This can expedite the mobility between theDUs.

FIGS. 35 to 37 illustrate another sequence on the mobility for the PDCPacknowledgement using HARQ-ACK. FIGS. 35 to 37 are connected acrosslocations of borders BL3536 and BL3637. In the example of FIGS. 35 to37, the UE determines the mobility. In FIGS. 35 to 37, the same stepnumbers are assigned to the same Steps as those in FIGS. 32 to 34, andthe common description thereof is omitted.

In Step ST2410 of FIG. 35, the UE determines the mobility using themeasurement result of Step ST1904. In the example of FIGS. 35 to 37, theDU #1 is switched to the DU #2. In Steps ST2411 and ST2412, the UEtransmits, to the CU, a request for the mobility between the DUs. The UEtransmits, to the DU #1, the request for the mobility between the DUs inStep ST2411, and the DU #1 transmits, to the CU, the request for themobility between the DUs in Step ST2412.

The sequence illustrated in FIGS. 35 to 37 enables reduction in theamount of signaling for notifying the measurement result.

The third embodiment may be applied to the PDCP-PDU having no PDCPsequence number. In the previous description, for example, a particularnumber may be assigned to the PDCP sequence number, and then theparticular number may be assigned to the PDCP-PDU having no PDCPsequence number. The PDCP-PDU having no PDCP sequence number may be, forexample, the PDCP control PDU. This can enhance the reliability intransmission of the PDCP control PDU for the mobility between the DUs.

The methods described in the third embodiment may be applied to caseswithout any mobility. For example, upon excess of the maximum HARQretransmission times, the DU may notify the CU of the PDCP sequencenumber of the PDCP-PDU included in a transport block in which themaximum HARQ retransmission times has been exceeded. The CU mayretransmit the PDCP-PDU with the PDCP sequence number to the DU. Thiscan prevent the packet loss due to the excess of the number of HARQretransmissions, and expedite retransmission of the PDCP-PDU.

One example of the detailed operations is hereinafter disclosed to applythe methods described in the third embodiment to the notification of thePDCP sequence number from the DU to the CU upon excess of the maximumHARQ retransmission times.

The MAC layer of the DU may obtain the RLC sequence number including thetransport block in which the maximum HARQ retransmission times has beenexceeded, using the HARQ process number upon transmission of thetransport block in which the maximum HARQ retransmission times has beenexceeded, and information for associating the RLC sequence number withthe HARQ process number.

The RLC layer of the DU obtains the PDCP sequence number of the PDCP-PDUincluding the transport block in which the maximum HARQ retransmissiontimes has been exceeded, using the information on the RLC sequencenumber notified from the MAC layer of the DU, and information forassociating the PDCP sequence number with the RLC sequence number. TheRLC layer of the DU notifies the CU of the PDCP sequence number.

The methods described in the third embodiment may be applied to themobility between secondary base stations. The mobility between secondarybase stations is applicable by reading the CU, the source DU, and thetarget DU in the third embodiment as a master base station, a sourcesecondary base station, and a target secondary base station,respectively. This can ensure the low latency and the high reliabilityin the mobility between the secondary base stations.

The methods described in the third embodiment may be applied to themobility while the C-Plane data is forwarded. The C-Plane data may be,for example, NAS data. The forwarding of the C-Plane data differs fromthat in the U-Plane in that the CU and the UE do not have any New ASlayer. In the example of FIG. 31, the CU 801 does not have the New ASlayer 807, and the high-level network device 805 directly transmits thepacket 806 to the PDCP 2201. Further, the UE 804 does not have the NewAS layer 823, and the PDCP layer 2220 directly forwards the PDCP-SDU2225 to the upper layer 824.

Application of the methods described in the third embodiment to themobility while the C-Plane data is forwarded can, for example, enhancethe reliability in transmission of NAS data upon occurrence of themobility between the DUs.

Although the third embodiment describes the downlink communication, thethird embodiment may be applied to the uplink communication.Specifically, application to the uplink communication is possible byreading the PDCP layer of the CU as the PDCP layer of the UE, readingthe RLC layer and the MAC layer of the DU as the RLC layer and the MAClayer of the UE, respectively, and reading the MAC layer of the UE asthe MAC layer of the DU. This can ensure the reliability in the uplinkcommunication without packet duplication.

In the third embodiment, the source DU may forward the PDCP-PDU to thetarget DU. The PDCP-PDU may be the PDCP-PDU including the transportblock data having no HARQ acknowledgement in the source DU. Since thesource DU forwards, to the target DU, the data the CU forwards to thetarget DU, the amount of data to be forwarded between the CU and the DUcan be reduced.

An interface between the DUs may be provided for the forwarding from thesource DU to the target DU. The source DU may use the interface betweenthe DUs to forward the PDCP-PDU to the target DU. This enables thecommunication between the DUs.

In the interface between the DUs, the methods defined in the X2/Xninterface may be applied as the interface between the DUs. The PDCP-PDUmay be forwarded, for example, by newly providing a message indicatingMOBILITY DATA TRNSFER. Consequently, the complexity in designing theinterface between the DUs can be avoided. Since the same processing asthat for the communication between the base stations in the DC isapplicable, the amount of processing in the base stations can bereduced.

The methods described in the third embodiment may be applied to the basestations and the UEs that communicate using the RLC-AM. This canexpedite the retransmission control more than that through theconventional ARQ with the RLC.

The methods described in the third embodiment can ensure the reliabilityand the low latency while reducing the use of resources through packetduplication.

The third embodiment provides, for example, the following configuration.

Provided is a communication system including a communication terminaldevice, and a base station device configured to perform radiocommunication with the communication terminal device. Specifically, thebase station device includes: a plurality of distributed units (DUs)that transmit and receive radio signals; and a central unit (CU) thatcontrols the plurality of DUs. The CU obtains, from a DU connected tothe communication terminal device, delivery complete information on apacket that has already been delivered to the communication terminaldevice. The CU transmits, from the DU or another DU to the communicationterminal device, a packet that is not indicated in the delivery completeinformation.

Here, the third embodiment discloses an example where the deliverycomplete information is a sequence number of a packet whose deliverycompletion has been notified from the communication terminal device. Thedelivery complete information is not limited to this example.

The configuration can be variously modified based on the disclosure andthe suggestion of the Description including the third embodiment. Theconfiguration and the modified configuration can solve the problems, andproduce the advantages.

First Modification of Third Embodiment

The third embodiment describes an example of the PDCP acknowledgementusing HARQ-ACK in Option 2 of the CU-DU split. The PDCP acknowledgementusing HARQ-ACK may be applied to Option 3-1 of the CU-DU split.

The DU notifies the CU of information on the RLC sequence number. The DUnotifies the CU of the sequence number of the RLC-PDU with HARQacknowledgement in its own DU, as the RLC sequence number. The CUtransmits, to the DU, an RLC-PDU that does not correspond to the RLCsequence number.

The first modification differs from the third embodiment in using theRLC sequence number instead of the PDCP sequence number.

FIG. 38 illustrates a configuration for RLC acknowledgement usingHARQ-ACK, in Option 3-1 of the CU-DU split. In FIG. 38, the same numbersare assigned to the same blocks as those in FIG. 31, and the commondescription thereof is omitted.

In FIG. 38, an RLC-L layer 2501 of the DU #1 (DU 802) obtains the RLCsequence number from the RLC-PDU obtained from an RLC-H layer 2502 ofthe CU 801. The RLC-L layer 2501 forwards the RLC-PDU to the MAC layer2203.

In FIG. 38, the MAC layer 2203 obtains the RLC sequence number from theRLC-PDU. The MAC layer 2203 transmits the transport block data generatedusing the RLC-PDU to the UE 804, and manages, in association with theRLC sequence number, the HARQ process number used for transmitting thetransport block data.

In FIG. 38, assume that a MAC layer 2210 of the UE 804 accuratelyreceives the user data (the transport block generated using an RLC-PDU2504) from the MAC layer 2203 of the DU #1. The MAC layer 2210 notifiesthe MAC layer 2203 of the DU #1 of HARQ-ACK information 2205.

In FIG. 38, the MAC layer 2203 of the DU #1 obtains the HARQ processnumber with HARQ-ACK from the HARQ-ACK information 2205. The MAC layer2203 finds the RLC sequence number with acknowledgement, using the HARQprocess number and the association between the HARQ process number andthe RLC sequence number. The MAC layer 2203 transmits the RLC sequencenumber to the RLC-L layer 2501 as an RLC sequence number notification2206.

In FIG. 38, the RLC-L layer 2501 and the MAC layer 2203 may update theRLC sequence number with acknowledgement, using the HARQ-ACK informationfrom the MAC layer 2210 of the UE 804 at any time. This enables the DU#1 to promptly notify the CU of the RLC sequence number upon occurrenceof the mobility between the DUs. Alternatively, the update may beperformed upon occurrence of the mobility between the DUs. This canreduce the amount of processing in the RLC-L layer 2501 and the MAClayer 2203.

In FIG. 38, assume the occurrence of the mobility between the DUs,specifically, from the DU #1 to the DU #2 when the RLC sequence numberwith acknowledgement is m. Upon occurrence of the mobility between theDUs, specifically, from the DU #1 to the DU #2, the RLC-L layer 2501transmits the RLC sequence number m with acknowledgement to the RLC-Hlayer 2502 of the CU 801 as an RLC sequence number notification 2503.The Fs interface 814 is used for the transmission.

In FIG. 38, the RLC-H layer 2502 transmits the RLC-PDU 2504 with an RLCsequence number (for example, m+1) having no acknowledgement to the DU#2 (DU 803), using the RLC sequence number notification 2503. The Fsinterface 815 is used for the transmission.

The configuration illustrated in FIG. 38 enables the RLC-H layer 2502 topromptly retransmit, to the UE 804 using the DU #2, the RLC-PDU that theUE cannot accurately receive upon occurrence of the mobility from the DU#1 to the DU #2.

The other detailed operations are identical to those in the thirdembodiment. Thus, the description is omitted. In Option 3-1 of the CU-DUsplit, application of the methods similar to those in the thirdembodiment produces the same advantages as those in the thirdembodiment.

The first modification of the third embodiment even with application ofOption 3-1 of the CU-DU split can ensure the reliability and the lowlatency while reducing the use of resources through packet duplication.

Fourth Embodiment

In a base station configuration using the DC, a source secondary basestation forwards, to a master base station, a downlink PDCP-PDU with noacknowledgement in the UE upon occurrence of the mobility of thesecondary base station, for example, upon change in the secondary basestation. The master base station forwards the PDCP-PDU to a targetsecondary base station (see Non-Patent Document 1).

The aforementioned method, however, causes a problem of constricting thebandwidth of an interface between base stations due to forwarding ofdata from the source secondary base station to the master base stationupon occurrence of the mobility.

The fourth embodiment discloses a method for solving such a problem.

Upon occurrence of the mobility of the secondary base station, themaster base station forwards, to a target secondary base station, aPDCP-PDU with no acknowledgement to the UE. The target secondary basestation transmits or retransmits the PDCP-PDU to the UE.

The master base station obtains information on the PDCP-PDU withacknowledgement in the UE, using a PDCP status report transmitted fromthe UE.

The PDCP status report may be a periodic PDCP status report described inNon-Patent Document 20 (3GPP TS36.323 v14.2.0). Consequently, the amountof signaling in the mobility can be reduced.

Upon occurrence of the mobility, the UE may transmit the PDCP statusreport to the master base station. The transmission may be routedthrough the source secondary base station. Transmission of the PDCPstatus report from the UE enables the master base station to obtain thelatest PDCP-PDU acknowledgement status that the UE understands uponoccurrence of the mobility. This can reduce unnecessary retransmissionfrom the master base station to the UE through the target secondary basestation, for example, retransmission of the PDCP-PDU withacknowledgement in the UE immediately before the mobility. This canprevent constricting the bandwidth of an interface between basestations.

Transmission of the PDCP status report from the UE in the mobility maybe determined in a standard. For example, upon receipt of the RRCconnection reconfiguration from the master base station, the UE maytransmit the PDCP status report. Consequently, the amount of signalingnecessary for transmitting the PDCP status report can be reduced.

Alternatively, the UE may transmit the PDCP status report in themobility, using polling from the master base station to the UE as atrigger. Consequently, the complexity in designing the PDCP layer can beavoided.

FIG. 39 is a sequence diagram illustrating the mobility betweensecondary base stations using a PDCP status report from the UE. FIG. 39illustrates an example of switching the secondary base station from theSgNB #1 to the SgNB #2.

In Step ST2601 of FIG. 39, the MgNB forwards user data to the SgNB #1.In Step ST2602, the SgNB #1 transmits the user data to the UE.

In Step ST2603 of FIG. 39, the MgNB notifies the SgNB #2 of an SgNBaddition request. In Step ST2604, the SgNB #2 returns an acknowledgementto the MgNB in response to Step ST2603. In Step ST2605, the MgNBnotifies the SgNB #1 of a request for SgNB release.

In Step ST2607 of FIG. 39, the MgNB notifies the UE of the RRCconnection reconfiguration. The notification includes informationindicating that the secondary base station is switched from the SgNB #1to the SgNB #2. The notification may include the RRC parameter for theSgNB #2. In Step ST2608, the UE notifies the MgNB of completion of theRRC connection reconfiguration.

In Step ST2609 of FIG. 39, the UE transmits the PDCP status report tothe MgNB. The report includes PDCP-PDU acknowledgement information inthe UE. The UE may transmit the PDCP status report to the MgNB throughthe SgNB #1.

In Step ST2610 of FIG. 39, the MgNB notifies the SgNB #2 of completionof the SgNB reconfiguration. In Step ST2611, a random access procedureis performed between the UE and the SgNB #2, and then the UE isconnected to the SgNB #2.

The MgNB forwards the user data to the SgNB #2 in Step ST2612 of FIG.39, and the SgNB #2 transmits the user data to the UE in Step ST2613.The user data may be the PDCP-PDU having no acknowledgement in the UEand included in the PDCP status report of Step ST2609.

In Step ST2614 of FIG. 39, the MgNB notifies the SgNB #1 of the UEcontext release to terminate a series of sequences on the mobility.

The fourth embodiment may be applied to the mobility between the DUs.This can prevent constricting the bandwidth of the interface between theCU and the DU, for example, the Fs interface. Specifically, applicationto the mobility between the DUs is possible by reading the MgNB as theCU and reading the SgNB as the DU.

The fourth embodiment may be applied to transmission and reception ofthe C-Plane data. For example, the fourth embodiment may be applied tothe C-Plane data in the mobility between the DUs. The C-Plane data maybe, for example, NAS data. This can ensure the efficient transmissionand the reliability in the mobility between the DUs during transmissionof the C-Plane data.

FIGS. 40 and 41 illustrate a sequence diagram of the mobility betweenDUs using a PDCP status report from the UE. FIGS. 40 and 41 areconnected across a location of a border BL4041. FIGS. 40 and 41illustrate an example of switching from the DU #1 to the DU #2. In FIGS.40 and 41, the same step numbers are assigned to the same Steps as thosein FIGS. 32 to 34, and the common description thereof is omitted.

In Step ST2701 of FIG. 41, the UE transmits the PDCP status report tothe DU #1. In Step ST2702, the DU #1 forwards the PDCP status report tothe CU. The PDCP status report illustrated in Steps ST2701 and ST2702may be identical to that of Step ST2609 in FIG. 39.

In Steps ST2703 and ST2704 of FIG. 41, the CU transmits the user data tothe UE through the DU #2. Steps ST2703 and ST2704 may handle thePDCP-PDU having no acknowledgement in the UE, similarly as Steps ST2612and ST2613 of FIG. 39.

The fourth embodiment can prevent constricting of the bandwidth of aninterface between the base stations and/or the interface between the CUand the DU, upon occurrence of the mobility.

The fourth embodiment provides, for example, the followingconfiguration.

Provided is a communication system including a communication terminaldevice, and a plurality of base station devices configured to performradio communication with the communication terminal device.Specifically, the plurality of base station devices include a masterbase station device and a secondary base station device which configurebearers for the communication terminal device. When the secondary basestation device that communicates with the communication terminal deviceis changed from a first secondary base station device to a secondsecondary base station device, the master base station device obtains,from the communication terminal device, delivery information on a packetthat has already been transmitted from the first secondary base stationdevice to the communication terminal device. The master base stationdevice transmits, from the second secondary base station device or themaster base station device to the communication terminal device, apacket that is not indicated in the delivery information.

The fourth embodiment also provides, for example, the followingconfiguration.

Provided is a communication system including a communication terminaldevice, and a base station device configured to perform radiocommunication with the communication terminal device. Specifically, thebase station device includes: a plurality of distributed units (DUs)that transmit and receive radio signals; and a central unit (CU) thatcontrols the plurality of DUs. The CU obtains, from a DU connected tothe communication terminal device, delivery complete information on apacket that has already been delivered to the communication terminaldevice. The CU transmits, from the DU or another DU to the communicationterminal device, a packet that is not indicated in the delivery completeinformation.

Here, the fourth embodiment discloses an example where the CU obtainsthe delivery complete information based on a packet data convergenceprotocol (PDCP) status report notified from the communication terminaldevice. The delivery complete information is not limited to thisexample.

The configuration can be variously modified based on the disclosure andthe suggestion of the Description including the fourth embodiment. Theconfiguration and the modified configuration can solve the problems, andproduce the advantages.

First Modification of Fourth Embodiment

The first modification discloses another method for solving the problemsin the fourth embodiment.

A source secondary base station forwards the RLC-PDU to a master basestation. The master base station forwards the RLC-PDU to a targetsecondary base station.

The source secondary base station may forward, to the target secondarybase station through the master base station, the RLC-PDU with noacknowledgement from the UE. The downlink data may be forwarded. Thiscan reduce the amount of forwarding between the base stations.

The source secondary base station may forward an RLC control PDU to thetarget secondary base station through the master base station. Sincethis eliminates the need for the target secondary base station toregenerate the RLC control PDU, the amount of processing in the targetsecondary base station can be reduced.

The source secondary base station may forward, to the target secondarybase station through the master base station, an RLC control PDU inwhich the PDCP-PDU assembly procedure is being performed. The uplinkdata may be forwarded. Even when this causes the target secondary basestation to miss the RLC-PDU, retransmission of the PDCP-PDU from the UEcan be prevented.

The source secondary base station may forward an RLC state variable tothe target secondary base station through the master base station. TheRLC state variable may be a variable described in 7.1 of Non-PatentDocument 21 (3GPP TS 36.322 v14.0.0). This can ensure the continuity inthe RLC entity before and after the mobility. Thus, processing in thecorresponding UE is facilitated.

The source secondary base station may forward, to the target secondarybase station through the master base station, a value of a timer to beused in the RLC entity. The value of the timer may be a value describedin 7.3 of Non-Patent Document 21 (3GPP TS 36.322 v14.0.0). This canmaintain smooth operations of the RLC entity.

The first modification may be applied to the mobility between the DUs.This can prevent constricting the bandwidth of the interface between theCU and the DU, for example, the Fs interface. Specifically, applicationto the mobility between the DUs is possible by reading the MgNB as theCU and reading the SgNB as the DU.

The first modification may also be applied to transmission and receptionof the C-Plane data. For example, the first modification may be appliedto the C-Plane data in the mobility between the DUs. The C-Plane datamay be, for example, NAS data. This can ensure the efficienttransmission and the reliability in the mobility between the DUs duringtransmission of the C-Plane data.

The first modification can prevent constricting of the bandwidth of aninterface between the base stations and/or the interface between the CUand the DU, upon occurrence of the mobility.

Fifth Embodiment

As disclosed in the first embodiment, the configuration for splittingthe CU and the DU is being studied in NR. A plurality of DUs may beprovided in the CU-DU split configuration. In such a case, which part ofthe gNB with the CU-DU split configuration in Option 2 performs routingto the DUs is a problem. Here, a method for performing routing to theDUs in the gNB is disclosed.

When the gNB has the CU-DU split configuration in Option 2, the CU ofthe gNB has routing functions between DUs. The CU of the gNB performsrouting to the DUs.

The PDCP in the CU of the gNB may have the routing functions betweenDUs. The PDCP in the CU of the gNB may perform routing to the DUs.

The routing functions between DUs include a function of determining arouting destination DU, and a function of transmitting data to thedetermined routing destination DU.

The routing functions between DUs may include a flow control function.The flow control function may be included in the function of determininga routing destination DU or in the function of transmitting data to thedetermined routing destination DU. Examples of the flow control includethe control over the data transmission timing such as start andtermination of data transmission, and the control over the datatransmission speed.

The routing functions between DUs may be set lower than the conventionalPDCP functions or the PDCP functions in the CU which are proposed in NR(Non-Patent Document 22: R3-170266).

When the PDCP in the CU has the routing functions for the split bearer,the routing functions between DUs may be set lower than the routingfunctions for the split bearer.

Setting the routing functions between DUs lower than the routingfunctions for the split bearer initiates the routing between DUs afterdata is split into the respective gNBs. It is possible to route, betweenDUs for each gNB, the data for the gNB. The complexity of providing therouting functions between DUs can be avoided.

In the routing functions between DUs, a routing destination DU may bedetermined and transmitted for each PDCP PDU. The routing destination DUmay be determined and transmitted for each of PDCP PDUs with apredetermined amount or a predetermined number. The routing destinationDU may be determined and transmitted at predetermined time intervals.The routing destination DU may be determined and transmitted for each ofPDCP PDUs with an amount meeting the demand from each DU. This canadjust the amount of PDCP PDUs to be transmitted to each DU and others.

This enables the gNB with the CU-DU split configuration in Option 2 toperform routing from the CU to the DU.

The receiver is provided with a function of routing data from each DU toa high-level function in the PDCP in the CU of the gNB. The function ofrouting data from each DU to the high-level function may be one of therouting functions between DUs.

The data from each DU is routed to the high-level function in order ofarrival as a function of the receiver. In other words, the data fromeach DU is transmitted one by one to the high-level function in order ofarrival.

As an alternative method, the data from each DU may be routed to thehigh-level function in order of PDCP sequence number (PDCP-SN). In otherwords, the data from each DU is routed in order of PDCP-SN andtransmitted one by one to the high-level function.

This enables the routing of the data from each DU of the gNB to thehigh-level function.

The CU in the gNB may be provided with a buffer for DU. The buffer maybe a buffer for communication on the interface between the CU and theDU. Application of a buffer for the routing between DUs or for therouting to the high-level function in the receiver enables data to beheld in the buffer. This can reduce a data overflow state and data loss,for example, in the presence of delay in data communication between theUE and the DU.

The buffer may be provided for each DU. Application of the buffer foreach DU can reduce a data overflow state for each DU and the data loss,according to a data communication state between the UE and the DU.Further, a data overflow state in another DU which is caused by delay indata communication in one of the DUs, and the data loss can be reduced.

The buffer for DU may be provided in the PDCP. This facilitatescoordinating processing with the routing functions between DUs.

The DU of the gNB may be provided with the buffer for each DU. Thebuffer may be provided in the RLC. The buffer may be a buffer forcommunication on the interface between the CU and the DU. Withapplication of the buffer for receiving data routed from the CU or fortransmitting data to the CU in the receiver, data can be held in thebuffer. This can reduce a data overflow state and data loss, forexample, in the presence of delay in data communication between the UEand the DU or delay in a function in the CU or the PDCP.

The RLC of each DU may notify the CU of information requesting thedownlink data. The RLC in each DU may notify the PDCP in the CU of theinformation.

Five examples of the information are disclosed below:

(1) an identifier of its own DU;

(2) a required amount of data;

(3) a required number of PDCP-PDUs;

(4) an amount of buffer allowance in its own DU; and

(5) combinations of (1) to (4) above.

The routing functions between DUs in the CU can determine to which DUdata should be transmitted in consideration of the information notifiedfrom each DU.

Each DU may notify the CU of information on successful transmission ortransmission failure resulted from the retransmission control in theRLC. Each DU may notify the information in association with theidentifier of its own DU. The PDCP in the CU can determine whether toperform retransmission, according to the information.

Each DU may notify the CU of information on successful transmission ortransmission failure resulted from the retransmission control (HARD) inthe MAC. Each DU may notify the information in association with theidentifier of its own DU. The PDCP in the CU can determine whether toperform retransmission, according to the information. The methodsdisclosed in, for example, the third embodiment may be applied to thismethod.

The PDCP in the CU determines again to which DU the data determined tobe retransmitted is routed using the routing functions between DUs, andtransmits the data to the determined routing destination DU. The datamay be transmitted not limited to the DU to which the data has beenpreviously transmitted but to an arbitrary DU. Enabling transmission toa DU different from the DU to which the data has been previouslytransmitted enables determination of a routing destination DU inconsideration of a communication state between the UE and the DU.

The CU of the gNB may have the functions of duplicating a packet anddetecting and removing redundant packets which are disclosed in thefirst embodiment, together with the routing functions between DUs. Thesefunctions may be the PDCP functions in the CU. The gNB can implement thefunction of duplicating a packet, and route, between DUs, the packetsduplicated by the gNB using the routing functions between DUs.

One of a method for determining the DU in the function of duplicating apacket disclosed in the first embodiment and the method for determininga routing destination DU in the routing functions between DUs should beapplied to a function of determining the DU that transmits theduplicated packets.

When the DU itself or the UE determines the DU that communicates theduplicated packets, the function of determining the routing destinationDU in the routing functions between DUs may be disabled. The function ofdetermining the routing destination DU in the routing functions betweenDUs may be bypassed or transparent. In the routing functions betweenDUs, a function of transmitting duplicated packets may be enabled forthe DU that communicates the duplicated packets determined by the DUitself or the UE.

This can configure the CU having both of the function of duplicating apacket and the routing functions between DUs.

The methods disclosed in the fifth embodiment enable the gNB tocommunicate with the UE using a plurality of DUs even when the pluralityof DUs are connected to the CU in the CU-DU split configuration inOption 2.

The fifth embodiment provides, for example, the following configuration.

Provided is a communication system including a communication terminaldevice and a base station device configured to perform radiocommunication with the communication terminal device. Specifically, thebase station device includes: a plurality of distributed units (DUs)that transmit and receive radio signals to and from the communicationterminal device; and a central unit (CU) that controls the plurality ofDUs. The CU has a function of routing downlink data addressed to thecommunication terminal device, in or lower than a packet dataconvergence protocol (PDCP).

The configuration can be variously modified based on the disclosure andthe suggestion of the Description including the fifth embodiment. Theconfiguration and the modified configuration can solve the problems, andproduce the advantages.

The Sixth Embodiment

In 3GPP, the dual connectivity (DC) using a split bearer (SB) is beingstudied as a technology of NR.

A master gNB (MgNB) responsible for performing routing to an MgNB or aSgNB using the SB is being proposed (Non-Patent Document 22: R3-170266).

Application of a configuration not using the PDCP of a secondary gNB(SgNB) for the UE performing the DC is being proposed in NR similarly asthe LTE (Non-Patent Document 9: TR38.804V1.0.0).

In the PDCP of the MgNB, data for the SgNB is routed to the SgNB, andtransmitted to the SgNB. The data transmitted to the SgNB is enteredinto the RLC in the SgNB.

When the gNB with the CU-DU split configuration in Option 2 is appliedas the SgNB of the SB, the following problems occur.

When the SgNB does not have the CU-DU split configuration as in theconventional LTE, the MgNB has only to transmit data to the SgNB. In NR,however, when the SgNB has the CU-DU split configuration, it is unclearto which part of the SgNB the MgNB should transmit data, the CU or theDU.

As previously described, the SgNB does not use the PDCP for the UEperforming the DC (TR38.804 v1.0.0). Thus, even though the MgNBtransmits data to the CU of the SgNB, the data cannot be entered intothe PDCP in the CU. Due to the absence of the RLC in the CU, theprocessing is impossible. Thus, transmission of data from the PDCP inthe CU to the RLC in the DU is impossible.

Even through the MgNB transmits data to the DU of the SgNB, it isunclear in which method and to which DU of another gNB the data shouldbe transmitted.

This disables transmission of the data from the MgNB to the SgNB.

Similarly, transmission of data from the SgNB to the MgNB in the uplinkis impossible. The description thereof is omitted because thetransmission path in the uplink is reverse to that described above.

Thus, a problem of failing to establish communication between the basestation side and the UE that communicates using the DUs of the SgNBoccurs.

The sixth embodiment discloses a method for solving such problems.

The SgNB is provided with the routing functions between DUs. The SgNBperforms routing between DUs. The CU of the SgNB is provided with therouting functions between DUs. The CU of the SgNB performs the routingbetween DUs. Data to be routed should be user data (U-plane data).Further, the data to be routed may be control data (C-plane data). Therouting of control data is applicable when a split bearer supports thecontrol data.

The SgNB that is a secondary gNB in the DC with the SB routes, betweenDUs, the PDCP-PDU transmitted from the MgNB.

As previously described, to which part of the CU of the SgNB the routingfunctions between DUs are provided is a problem. Here, three examples ofwhere the routing functions between DUs are provided are disclosed.

(1) The routing functions between DUs are provided inside the PDCP inthe CU of the SgNB.

(2) The routing functions between DUs are provided outside the PDCP inthe CU of the SgNB.

(3) A protocol stack having the routing functions between DUs isprovided outside the PDCP in the CU of the SgNB.

The details on (1) are disclosed.

The routing functions between DUs are provided inside the PDCP in the CUof the SgNB. Data forwarding between the MgNB and the SgNB is disclosed.The MgNB of the UE which is a connecting destination of the split bearerenters data into the PDCP of the SgNB. The CU of the SgNB executes onlythe routing functions between DUs for the PDCP. The CU may enable onlythe routing functions. The CU may prevent the other functions from beingexecuted. The other functions may be bypassed or transparent. The PDCPin the CU of the SgNB outputs data to each DU.

In the receiver, a function of routing data from each DU to the MgNB isprovided in the PDCP in the CU of the SgNB. The function of routing datafrom each DU to the MgNB may be one of the routing functions betweenDUs.

The data from each DU is routed to the MgNB in order of arrival as afunction of the receiver. In other words, the data from each DU istransmitted one by one to the MgNB in order of arrival.

As an alternative method, the data from each DU may be routed to theMgNB in order of PDCP sequence number (PDCP-SN). In other words, thedata from each DU is routed in order of PDCP-SN and transmitted one byone to the MgNB.

FIG. 42 illustrates an example architecture when the PDCP of the SgNB isprovided with the routing functions between DUs. FIG. 42 illustratesthat both the MgNB and the SgNB have the CU-DU split configurations inOption 2.

The MgNB includes a CU and DUs. The two DUs (the DUs #1 and #2) areconnected to the one CU. The CU includes the RRC, the New AS layer, andthe PDCP. Each of the DUs includes the RLC, the MAC, and the PHY.

The SgNB includes a CU and DUs. The two DUs (the DUs #1 and #2) areconnected to the one CU. The CU of the SgNB includes the RRC. Each ofthe DUs includes the RLC, the MAC, and the PHY. Since the SgNB is asecondary gNB of the UE performing the DC, the CU does not include theNew AS layer and the PDCP in the conventional configuration. In theexample of FIG. 42, however, the PDCP is provided in the CU.

The routing functions between DUs are provided into the PDCP in the CUin the SgNB. In the example of FIG. 42, the routing functions betweenDUs are also provided into the PDCP in the CU in the MgNB.

An interface should be provided between the CU of the MgNB and the CU ofthe SgNB for data communication between the MgNB and the SgNB. Theinterface may be a new interface, or Xn or Xx that is being studied in3GPP as an interface between gNB s.

The MgNB of the UE which is a connecting destination of the split bearerenters data into the PDCP of the SgNB through the interface. The PDCPconfigured in the CU of the SgNB executes only the routing functionsbetween DUs. The PDCP outputs the data from the MgNB to each DU with therouting functions between DUs.

In the receiver, the data from each DU is routed to the MgNB with therouting functions between DUs that are configured in the PDCP in the CUof the SgNB. For example, the SgNB routes and transmits the data fromeach DU to the MgNB in order of arrival.

Since such a method enables the SgNB to use the PDCP that is an existingprotocol stack, the gNB can be easily configured. The gNB with the CU-DUsplit configuration in Option 2 is easily used as a secondary gNB in theDC.

The CU of the SgNB may have the functions of duplicating a packet anddetecting and removing redundant packets which are disclosed in thefirst embodiment. The functions may be provided in the PDCP configuredin the CU. Thereby, a packet can be duplicated in the SgNB.

The CU of the SgNB may have the functions together with the routingfunctions between DUs. These functions may be the PDCP functions in theCU. The gNB having, in the PDCP of the CU, both of the function ofduplicating a packet and the routing functions between DUs as disclosedin the fifth embodiment may be used as an SgNB. When the gNB is used asan SgNB of the UE dependent on the DC, the function of duplicating apacket and the routing functions between DUs should be enabled.

This enables the SgNB to implement, using a plurality of DUs, thefunction of duplicating a packet and the function of detecting andremoving redundant packets. The MgNB does not have to execute thefunction of duplicating a packet to be performed by the SgNB using aplurality of DUs. Thus, increase in the data communication volumebetween the MgNB and the SgNB can be prevented.

The details on (2) are disclosed.

The routing functions between DUs are provided outside the PDCP in theCU of the SgNB. The RRC of the SgNB may have the routing functions. Thedata forwarding between the MgNB and the SgNB is disclosed. The MgNB ofthe UE which is a connecting destination of the split bearer enters datainto the CU of the SgNB. The CU of the SgNB executes only the routingfunctions between DUs. The CU may enable only the routing functions. TheCU may prevent the other functions from being executed. The otherfunctions may be bypassed or transparent. The CU of the SgNB outputsdata to each DU.

In the receiver, a function of routing the data from each DU to the MgNBis provided outside the PDCP in the CU of the SgNB. The function ofrouting the data from each DU to the MgNB may be one of the routingfunctions between DUs.

The methods previously disclosed in (1) should be applied as functionsof the receiver.

FIG. 43 illustrates an example architecture when the routing functionsbetween DUs are provided outside the PDCP in the CU of the SgNB. FIG. 43illustrates that both the MgNB and the SgNB have the CU-DU splitconfigurations in Option 2.

The MgNB includes a CU and DUs. The two DUs (the DUs #1 and #2) areconnected to the one CU. The CU includes the RRC, the New AS layer, andthe PDCP. Each of the DUs includes the RLC, the MAC, and the PHY.

The SgNB includes a CU and DUs. The two DUs (the DUs #1 and #2) areconnected to the one CU. The CU of the SgNB includes the RRC. Each ofthe DUs includes the RLC, the MAC, and the PHY. Since the SgNB is asecondary gNB of the UE performing the DC, the CU does not include theNew AS layer and the PDCP.

The routing functions between DUs are provided outside the PDCP in theCU of the SgNB. In the example of FIG. 43, the routing functions betweenDUs are provided outside the PDCP in the CU of the MgNB.

A gNB may have the routing functions between DUs outside the PDCP of thegNB. Although the routing functions between DUs are provided into thePDCP of the MgNB in the example of FIG. 42, the routing functionsbetween DUs may be provided outside the PDCP of the MgNB herein.

Similarly as (1), an interface should be provided between the CU of theMgNB and the CU of the SgNB for data communication between the MgNB andthe SgNB.

The MgNB of the UE which is a connecting destination of the split bearerenters data into the routing functions between DUs that are providedoutside the PDCP in the CU of the SgNB, through the interface. Theentered data is output to each DU with the routing functions betweenDUs.

In the receiver, the data from each DU is routed to the MgNB with therouting functions between DUs that are provided outside the PDCP in theCU of the SgNB. For example, the SgNB routes and transmits the data fromeach DU to the MgNB in order of arrival.

When a split bearer is executed, application of such a method enablesthe SgNB to have the configuration not using the PDCP, similarly as thatusing the conventional split bearer. This facilitates functionalextension in the SgNB.

When the RRC of the SgNB has the routing functions between DUs, therouting functions between DUs can be executed by merely adding afunction to the RRC. Thus, the gNB can be easily configured. When theRRC of the SgNB has the routing functions between DUs, collectiverouting using a plurality of bearers is possible. Thus, the amount ofprocessing can be reduced.

The CU of the SgNB may have the functions of duplicating a packet anddetecting and removing redundant packets which are disclosed in thefirst embodiment. The functions may be provided outside the PDCPconfigured in the CU. Thereby, the packet can be duplicated in the SgNB.

The CU of the SgNB may have the functions together with the routingfunctions between DUs. These functions may be functions outside the PDCPin the CU. The gNB having, outside the PDCP in the CU, both of thefunction of duplicating a packet and the routing functions between DUsmay be used as the SgNB. When the gNB is used as an SgNB of the UEdependent on the DC, the function of duplicating a packet and therouting functions between DUs should be enabled.

This enables the SgNB to implement, using a plurality of DUs, thefunctions of duplicating a packet and detecting and removing redundantpackets. The MgNB does not have to execute the function of duplicating apacket to be performed by the SgNB using a plurality of DUs. Thus,increase in the data communication volume between the MgNB and the SgNBcan be prevented.

The details on (3) are disclosed.

A protocol stack having the routing functions between DUs (may bereferred to as an “RP”) is provided outside the PDCP in the CU of theSgNB. The RP may be provided separately from the PDCP. The RP may be setlower than the PDCP. The RP performs the routing between DUs in theSgNB.

The data forwarding between the MgNB and the SgNB is disclosed. The MgNBof the UE which is a connecting destination of the split bearer entersdata into the CU of the SgNB. The MgNB should enter the data in the RPin the CU of the SgNB. With the routing functions between DUs providedin the RP, the data is routed between DUs and output to each of the DUs.

In the receiver, a function of routing the data from each DU to the MgNBis provided in the RP in the CU of the SgNB. The function of routing thedata from each DU to the MgNB may be one of the routing functionsbetween DUs.

The methods previously disclosed in (1) should be applied as functionsof the receiver.

FIG. 44 illustrates an example architecture when the protocol stackhaving the routing functions between DUs are provided outside the PDCPin the CU of the SgNB. FIG. 44 illustrates that both the MgNB and theSgNB have the CU-DU split configurations in Option 2.

The MgNB includes a CU and DUs. The two DUs (the DUs #1 and #2) areconnected to the one CU. The CU includes the RRC, the New AS layer, andthe PDCP. Each of the DUs includes the RLC, the MAC, and the PHY.

The SgNB includes a CU and DUs. The two DUs (the DUs #1 and #2) areconnected to the one CU. The CU of the SgNB includes the RRC. Each ofthe DUs includes the RLC, the MAC, and the PHY. Since the SgNB is asecondary gNB of the UE performing the DC, the CU does not include theNew AS layer and the PDCP.

The RP is provided outside the PDCP in the CU of the SgNB. The routingfunctions between DUs are provided in the RP. In the example of FIG. 44,the RP is provided outside the PDCP in the CU of the MgNB. The RP hasthe routing functions between DUs.

A gNB may have the RP outside the PDCP of the gNB, and the RP may havethe routing functions between DUs. Although the routing functionsbetween DUs are provided outside the PDCP of the MgNB in the example ofFIG. 43, the RP may be provided outside the PDCP of the MgNB, and the RPmay have the routing functions between DUs herein.

The RP may be a protocol stack common to gNBs which has both of therouting functions between DUs for MgNB and the routing functions betweenDUs for SgNB. Alternatively, the RP may be a protocol stack common togNBs with the integrated routing functions between DUs for MgNB andSgNB. For example, the PDCP may enter data into the RP in thetransmitter, and the RP may transmit the data to the PDCP in thereceiver.

Data is transmitted and received between the PDCP and the RP in theMgNB. Data is transmitted and received between the PDCP in the MgNB andthe RP in the SgNB.

A gNB may be an MgNB for a UE, and may be an SgNB for another UE. Thus,the RP common to gNBs enables execution of the routing between DUs evenwhen a gNB is used as an MgNB or an SgNB.

Similarly as (1), an interface should be provided between the CU of theMgNB and the CU of the SgNB for data communication between the MgNB andthe SgNB.

The MgNB of the UE which is a connecting destination of the split bearerenters data into the RP provided outside the PDCP in the CU of the SgNB,through the interface. The entered data is output to each DU with therouting functions between DUs.

In the receiver, the data from each DU is routed to the MgNB with therouting functions between DUs in the RP which are provided outside thePDCP in the CU of the SgNB. For example, the SgNB routes and transmitsthe data from each DU to the MgNB in order of arrival.

Application of the protocol stack having the routing functions betweenDCs enables a gNB to be configured without influencing the otherprotocol stacks in the gNB. Thus, the SgNB can be easily configured.

Since application of the RP common to gNBs eliminates the need forproviding different designs, implementations, or settings for the MgNBand the SgNB, the complexity can be avoided.

The RP may have the functions of duplicating a packet and detecting andremoving redundant packets which are disclosed in the first embodiment.The RP having the functions of duplicating a packet and detecting andremoving redundant packets enables a gNB to be configured withoutinfluencing the other protocol stacks.

Since the functions of duplicating a packet and detecting and removingredundant packets which are provided in the RP common to gNBs eliminatesthe need for providing different designs, implementations, or settingsfor the MgNB and the SgNB, the complexity can be avoided.

The CU of the SgNB may have the functions of duplicating a packet anddetecting and removing redundant packets which are disclosed in thefirst embodiment.

The RP configured in the CU may have the functions. Thereby, the packetcan be duplicated in the SgNB.

The CU of the SgNB may have the functions together with the routingfunctions between DUs. These functions may be functions in the RP of theCU. The gNB having, inside the RP in the CU, both of the function ofduplicating a packet and the routing functions between DUs may be usedas the SgNB. When the gNB is used as an SgNB of the UE dependent on theDC, the function of duplicating a packet and the routing functionsbetween DUs should be enabled.

This enables the SgNB to implement, using a plurality of DUs, thefunctions of duplicating a packet and detecting and removing redundantpackets. The MgNB does not have to execute the function of duplicating apacket to be performed by the SgNB using a plurality of DUs. Thus,increase in the data communication volume between the MgNB and the SgNBcan be prevented.

The CU in a gNB may be provided with a buffer for DU. The DU of a gNBmay be provided with a buffer for each DU. The methods disclosed in thefifth embodiment should be applied. The same advantages are produced.

Each DU may notify the CU of information requesting the downlink data,information on the RLC retransmission control, and information on theMAC retransmission control. The methods disclosed in the fifthembodiment should be applied. The same advantages are produced.

The fifth embodiment discloses that the PDCP in the CU determines againwhich DU the data determined to be retransmitted is routed to using therouting functions between DUs, and transmits the data to the determinedrouting destination DU. In the sixth embodiment, the CU or the protocolstack in the CU which is provided with the routing functions between DUsshould perform these processes. The same advantages are produced.

The SgNB may notify the MgNB of information indicating the presence orabsence of the CU-DU split configuration of its own SgNB.

The SgNB may also notify the MgNB of information on the routingfunctions between DUs of its own SgNB. Examples of the information onthe routing functions between DUs include the presence or absence of therouting functions between DUs and configuration information on where therouting functions between DUs are provided.

The SgNB may notify the MgNB of these pieces of information upon settingof the DC. For example, upon SgNB addition, the SgNB may notify the MgNBof the information. The SgNB may notify the MgNB of the information byincluding the information in signaling for notifying a response to therequest for the SgNB addition.

Alternatively, upon SgNB modification, the SgNB may notify the MgNB ofthe information. The SgNB may notify the MgNB of the information byincluding the information in signaling for notifying a response to therequest for the SgNB modification.

As such, the SgNB notifies the MgNB of the information indicating thepresence or absence of the CU-DU split configuration and the informationon the routing functions between DUs, so that the MgNB can recognizethese. The MgNB may perform the routing for the SB in consideration ofthese.

The information may be notified between gNBs not upon setting of the DCbut, for example, upon setting up an interface between gNBs. Theinformation may be notified to the other gNBs by including, in a set-upmessage, the information indicating the presence or absence of the CU-DUsplit configuration of its own gNB and information on the routingbetween DUs.

Alternatively, the information may be notified upon update of the gNBsetting. The information may be notified to the other gNBs by including,in an update message for the gNB setting, the information indicating thepresence or absence of the CU-DU split configuration of its own gNB andthe information on the routing between DUs.

The other gNBs may be surrounding gNBs.

Such notifications of the information, for example, upon setting up theinterface or upon update of the gNB setting enable consideration of theconfigurations of the surrounding gNBs not only in the DC but in theother services.

Information to be notified between the MgNB and the SgNB is disclosed.The information may be information to be notified between the MgNB andthe SgNB when the SB is applied in the LTE. The information may beinformation to be notified through X2-U (Non-Patent Document 23:TS36.425).

The information to be notified from the MgNB to the SgNB includesinformation associated with the DL data. The information associated withthe DL data is, for example, a sequence number assigned through theinterface between the MgNB and the SgNB. When Xn or Xx is provided, thesequence number assigned through the interface should be the informationassociated with the DL data. Examples of the sequence number includeXn-U SN and Xx-U SN.

The information to be notified from the SgNB to the MgNB is informationon a transmission state of the DL data. The information on atransmission state of the DL data is, for example, the highest PDCP-SNsuccessfully transmitted to the UE. The SgNB may notify the PDCP-SNsuccessfully transmitted to the UE for a predetermined duration as theinformation on a transmission state of the DL data. The SgNB may notifythe PDCP-SN failing in transmission to the UE for a predeterminedduration as the information on a transmission state of the DL data. TheSgNB may notify the first PDCP-SN to be notified, together with thePDCP-SN successfully transmitted or having failed in transmission for apredetermined duration. The PDCP-SN successfully transmitted or havingfailed in transmission for a predetermined duration may be notified asbitmap information. This can reduce the number of bits required for thenotification.

The information on a transmission state of the DL data may be of, forexample, the highest PDCP-SN successfully transmitted from the CU to theDU or the RLC in the DU. The SgNB may notify the PDCP-SN successfullytransmitted from the CU to the DU or the RLC in the DU for apredetermined duration as the information on a transmission state of theDL data. The SgNB may notify the PDCP-SN whose transmission from the CUto the DU or the RLC in the DU has failed for a predetermined durationas the information on a transmission state of the DL data. The SgNB maynotify the first PDCP-SN to be notified, together with the PDCP-SNsuccessfully transmitted or having failed in transmission for apredetermined duration. The PDCP-SN successfully transmitted or havingfailed in transmission for a predetermined duration may be notified asthe bitmap information. This can reduce the number of bits required forthe notification.

The information on a transmission state of the DL data may be of, forexample, the highest PDCP-SN successfully transmitted from the DU or theRLC in the DU to the UE. The SgNB may notify the PDCP-SN successfullytransmitted from the DU or the RLC in the DU to the UE for apredetermined duration as the information on a transmission state of theDL data. The SgNB may notify the PDCP-SN whose transmission from the DUor the RLC in the DU to the UE has failed for a predetermined durationas the information on a transmission state of the DL data. The SgNB maynotify the first PDCP-SN to be notified, together with the PDCP-SNsuccessfully transmitted or having failed in transmission for apredetermined duration. The PDCP-SN successfully transmitted or havingfailed in transmission for a predetermined duration may be notified asthe bitmap information. This can reduce the number of bits required forthe notification.

The DU may notify the CU of information on the highest PDCP-SNsuccessfully transmitted from the DU or the RLC in the DU to the UE,information on the PDCP-SN successfully transmitted from the DU or theRLC in the DU to the UE for a predetermined duration, and information onthe PDCP-SN whose transmission from the DU or the RLC in the DU to theUE has failed for a predetermined duration. Alternatively, the RLC inthe DU may notify the PDCP in the CU of these pieces of information. TheCU in the SgNB can notify the MgNB of these pieces of information.

Information to be notified from the SgNB to the MgNB includesinformation on a buffer size. Examples of such information includeinformation on a desired buffer size for a bearer to be the SB andinformation on the minimum desired buffer size for the UE dependent onthe SB.

The information to be notified from the SgNB to the MgNB includesinformation on data lost in the data communication between the MgNB andthe SgNB. The information on lost data is, for example, the sequencenumber assigned to the lost data through the interface between the MgNBand the SgNB. The other examples of the information on lost data includethe sequence number assigned to data lost first through the interfacebetween the MgNB and the SgNB, and the sequence number assigned to datalost last through the interface between the MgNB and the SgNB. The SgNBmay notify the number of series of the lost data sets.

This enables appropriate and efficient data transmission even when theMgNB transmits data to the PDCP in the CU of the SgNB.

FIGS. 45 to 47 illustrate an example sequence on the DC with the SBusing the SgNB with the CU-DU split configuration. FIGS. 45 to 47 areconnected across locations of borders BL4546 and BL4647.

The SgNB has the CU-DU split configuration in Option 2, and two DUs (theDUs #1 and #2) are connected to one CU. The MgNB also has the CU-DUsplit configuration in Option 2, and two DUs (the DUs #1 and #2) areconnected to one CU.

In Step ST6801, data is transmitted and received between the high-levelNW device and the MgNB CU.

In Step ST6802, the MgNB CU performs the routing between DUs. Forexample, when the PDCP has the routing functions between DUs, the PDCPperforms the routing between DUs. In the routing between DUs, a routingdestination DU is determined, and data is transmitted to the determinedrouting destination DU.

In Step ST6803, the MgNB CU transmits, to the MgNB DU #1, datadetermined to be routed to the MgNB DU #1. In Step ST6804, the MgNB DU#1 performs RLC, MAC, and PHY processes on the data received from theMgNB CU and transmits the processed data to the UE.

In Step ST6806, the MgNB CU transmits, to the MgNB DU #2, datadetermined to be routed to the MgNB DU #2. In Step ST6805, the MgNB DU#2 performs the RLC, MAC, and PHY processes on the data received fromthe MgNB CU and transmits the processed data to the UE.

When the UE performs transmission, operations reverse to these areperformed. In Step ST6804, the UE transmits data to the MgNB DU #1. InStep ST6803, the data received by the MgNB DU #1 is processed by thePHY, the MAC, and the RLC, and transmitted to the MgNB CU. Similarly, inStep ST6805 and ST6806, the UE transmits data to the MgNB CU through theMgNB DU #2. In Step ST6802, the MgNB CU performs the routing between DUsin the receiver. For example, the MgNB CU transmits, to a high-levelfunction, the data from the MgNB DU #1 and the data from the MgNB DU #2in order of arrival. The data from the UE is processed by the PDCP inthe MgNB CU, processed by the New AS layer, and transmitted to thehigh-level NW device in Step ST6801.

Even when the MgNB includes a plurality of DUs, these enablecommunication between the UE and the high-level NW device.

Next, execution of the DC with the SB is described. In Step ST6807, theMgNB determines to execute the DC for the UE.

In Step ST6808, the MgNB CU notifies the SgNB CU of an SgNB additionrequest message. Upon receipt of the message, the SgNB CU permits itsown gNB to be used as the SgNB.

Here in Step ST6809, the SgNB CU may determine the DU to be applied inthe DC.

In Step ST6810, the SgNB CU notifies the MgNB CU of an SgNB additionrequest response message. The SgNB CU includes, in this message, the DUto be applied in the DC that has been determined in Step ST6809, andgives the notification.

Upon receipt of the SgNB addition request response message, the MgNB CUnotifies the UE that performs the DC of the setting of the DC in StepST6811. Here, the MgNB CU should notify information on the DUs of theSgNB to be applied in the DC. Examples of the information include theidentifiers of the DUs #1 and #2, the resource to be used in the initialaccess (for example, random access), and the sequence information.

The message for the RRC connection reconfiguration should be used inthis notification. Consequently, the UE can identify which DU of theSgNB the DC is performed with, and access the DU performing the DC.

In Step ST6812, the UE notifies the MgNB CU of completion of the DCsetting. The message for completion of the RRC connectionreconfiguration should be used in this notification.

In Step ST6813, the MgNB CU notifies the SgNB CU of completion of theSgNB reconfiguration.

The DC setting with the SB is made in such a manner.

In Steps ST6814 and ST6815, the UE performs random access (RA) procedurewith the DUs #1 and #2 of the SgNB performing the DC. This enablesaccess between the UE and the DUs #1 and #2 of the SgNB performing theDC.

In Step ST6816, the MgNB CU routes data to be transmitted from thehigh-level NW device, to the MgNB and the SgNB for the SB. In StepST6821, the MgNB CU transmits, to the SgNB, the data routed to the SgNB.Specifically, the MgNB CU transmits the data to the CU of the SgNB.

In Step ST6827, the MgNB CU routes, between the DUs, the data routed tothe MgNB. With the routing between DUs in Step ST6827, the MgNB CUtransmits, to the MgNB DU #1 in Step ST6817, the data that the MgNB CUdetermines to route to the MgNB DU #1. In Step ST6818, the MgNB DU #1processes the received data and transmits the processed data to the UE.

With the routing between DUs in Step ST6827, the MgNB CU transmits, tothe MgNB DU #2 in Step ST6820, the data that the MgNB CU determines toroute to the MgNB DU #2. In Step ST6819, the MgNB DU #2 processes thereceived data and transmits the processed data to the UE.

In Step ST6822, the data transmitted to the SgNB CU is routed betweenthe DUs. Even when the routing functions between DUs are provided in thePDCP of the SgNB, only the routing functions between DUs are enabled.

With the routing between DUs in Step ST6822, the SgNB CU transmits, tothe SgNB DU #1 in Step ST6823, the data that the SgNB CU determines toroute to the SgNB DU #1. In Step ST6824, the SgNB DU #1 processes thereceived data and transmits the processed data to the UE.

With the routing between DUs in Step ST6822, the SgNB CU transmits, tothe SgNB DU #2 in Step ST6826, the data which the SgNB CU determines toroute to the SgNB DU #2. In Step ST6825, the SgNB DU #2 processes thereceived data and transmits the processed data to the UE.

When the UE performs transmission, operations reverse to these areperformed. Since the same methods as disclosed in Steps ST6802 to StepST6806 are applied to the MgNB side, the description thereof is omitted.

The SgNB is disclosed.

In Step ST6824, the UE transmits data to the SgNB DU #1. The datareceived by the SgNB DU #1 is processed by the PHY, the MAC, and theRLC, and transmitted to the SgNB CU in Step ST6823. Similarly, in StepsST6825 and ST6826, the UE transmits data to the SgNB CU through the SgNBDU #2.

In Step ST6822, the SgNB CU performs the routing between DUs in thereceiver. For example, the SgNB CU transmits, to the MgNB, the data fromthe SgNB DU #1 and the data from the MgNB DU #2 in order of arrival. InStep ST6821, the SgNB CU transmits the data from the UE to the MgNB CU.The data received by the MgNB CU is processed by the PDCP, processed bythe New AS layer, and transmitted to the high-level NW device.

The aforementioned information to be notified between the MgNB and theSgNB may be notified in the communication between the MgNB CU and theSgNB CU in Step ST6821.

Consequently, the gNB with the CU-DU split configuration in Option 2 canbe used as the secondary gNB to perform the DC with the SB.

The methods disclosed in the sixth embodiment enable the MgNB to forwarddata to the SgNB when the gNB with the CU-DU split configuration inOption 2 is applied as the SgNB for the DC with the SB. The methods alsoenable the CU of the SgNB to perform the routing between DUs.

Thus, communication using the gNB with the CU-DU split configuration inOption 2 as the SgNB for the DC with the SB is possible.

Further, application of the methods disclosed in the sixth embodiment tothe MgNB enables the routing between DUs in the MgNB.

Thus, communication using the gNB with the CU-DU split configuration inOption 2 as the MgNB for the DC with the SB is possible.

In the examples previously disclosed, the method for configuring therouting functions between DUs is consistent between the MgNB and theSgNB. This can facilitate the configuration of the gNB because therouting functions between DUs common to the gNBs can be provided.

As an alternative method, the methods for configuring the routingfunctions between DUs may differ between the MgNB and the SgNB. Theconfigurations of the routing functions between DUs may differ accordingto the other configurations of the gNB s.

When the gNB has a plurality of configurations of the routing functionsbetween DUs, the configuration of the routing functions between DUs maybe changed semi-statically or dynamically. The gNB should notify anothergNB or the gNB establishing the DC of the configuration information ofits own gNB on the routing functions between DUs as information on therouting functions between DUs.

Consequently, the routing functions between DUs can be changed accordingto the configuration of each gNB, a load state of the gNB, or theothers. The processing load of the routing between DUs in the gNB can beappropriately reduced.

The sixth embodiment provides, for example, the following configuration.

Provided is a communication system including a communication terminaldevice, and a plurality of base station devices configured to performradio communication with the communication terminal device. Theplurality of base station devices include a master base station deviceand a secondary base station device which configure bearers for thecommunication terminal device. Each of the master base station deviceand the secondary base station device includes: a plurality ofdistributed units (DUs) which transmit and receive radio signals to andfrom the communication terminal device; and a central unit (CU) whichcontrols the plurality of DUs. The master base station device receives,from a network device higher than the master base station device,downlink data addressed to the communication terminal device. The masterbase station routes the downlink data to the master base station deviceand the secondary base station device.

Here, the CU of the master base station device has: a function ofdetermining a routing destination DU of the master base station device;and a function of forwarding, to the determined routing destination DU,the downlink data routed to the master base station device. Here, the CUof the secondary base station device has: a function of determining arouting destination DU of the secondary base station device; and afunction of forwarding, to the determined routing destination DU, thedownlink data routed to the secondary base station device.

Alternatively, the CU of the master base station device may have: afunction of determining a routing destination DU of the master basestation device; a function of forwarding, to the determined routingdestination DU, the downlink data routed to the master base stationdevice; and a function of determining a routing destination DU of thesecondary base station device. In this case, the CU of the secondarybase station device has a function of forwarding the downlink datarouted to the secondary base station device, to the routing destinationDU of the secondary base station device that has been determined by themaster base station device.

Alternatively, the CU of the master base station device may have: afunction of determining a routing destination DU of the master basestation device; a function of forwarding, to the determined routingdestination DU, the downlink data routed to the master base stationdevice; a function of determining a routing destination DU of thesecondary base station device; and a function of forwarding, to therouting destination DU of the secondary base station device, thedownlink data routed to the secondary base station device.

The configuration can be variously modified based on the disclosure andthe suggestion of the Description including the sixth embodiment. Theconfiguration and the modified configuration can solve the problems andproduce the advantages.

The first modification of the sixth embodiment

The first modification discloses the other methods for solving theproblems described in the sixth embodiment, specifically, the problemswhen the gNB with the CU-DU split configuration in Option 2 is appliedas the SgNB for the SB.

The MgNB is provided with a function of determining a routingdestination DU of the SgNB. The SgNB is provided with a function oftransmitting data to the routing destination DU. The CU of the SgNB isprovided with a function of transmitting data to the routing destinationDU.

The function of determining a routing destination DU of the SgNB may beset lower than the conventional PDCP functions or the PDCP functionswhich are proposed in NR (Non-Patent Document 22: R3-170266).

The function of determining a routing destination DU of the SgNB is setlower than the routing functions for the SB. The function of determininga routing destination DU of the SgNB may be provided after the routingto the SgNB.

Three examples of a method for configuring, in the MgNB, the function ofdetermining a routing destination DU of the SgNB are hereinafterdisclosed:

(1) the function of determining a routing destination DU is providedinside the PDCP;

(2) the function of determining a routing destination DU is providedoutside the PDCP; and

(3) a protocol stack having the function of determining a routingdestination DU is provided outside the PDCP.

Three examples of a method for configuring, in the CU of the SgNB, afunction of transmitting data from the MgNB to a routing destination DUare hereinafter disclosed:

(1) the function of transmitting the data to a routing destination DU isprovided in the PDCP in the CU in the SgNB;

(2) the function of transmitting the data to a routing destination DU isprovided outside the PDCP in the CU in the SgNB; and

(3) a protocol stack having the function of transmitting the data to arouting destination DU is provided outside the PDCP in the CU in theSgNB.

The gNB may be provided with the routing functions between DUs.

When the gNB is the MgNB, the routing functions between DUs areconfigured for the SgNB. Among the routing functions between DUs of theSgNB, the function of determining a routing destination DU is enabled,whereas the function of transmitting data to the routing destination DUis disabled. As an alternative example when the gNB is the MgNB, amongthe routing functions between DUs of the SgNB, the functions other thanthe function of determining a routing destination DU may be bypassed ortransparent. Consequently, when the gNB is the MgNB, the MgNB canimplement the function of determining a routing destination DU of theSgNB.

When the MgNB has the CU-DU split configuration in Option 2, the routingfunctions between DUs may be configured for the MgNB. The MgNB canimplement the routing functions between DUs of its own gNB.

When the gNB is the SgNB, among the routing functions between DUs, thefunction of determining a routing destination DU is disabled, whereasthe function of transmitting data to the routing destination DU isenabled. As an alternative example when the gNB is the SgNB, among therouting functions between DUs, the functions other than the function oftransmitting data to a routing destination DU may be bypassed ortransparent. Consequently, when the gNB is the SgNB, the SgNB canimplement the function of transmitting data to a routing destination DU.

The MgNB determines a routing destination DU of the SgNB and transmitstransmission data to the SgNB by implementing the function ofdetermining the routing destination DU of the SgNB for the transmissiondata to the SgNB. The SgNB can transmit data to each DU by implementingthe function of transmitting, to a routing destination DU, datatransmitted from the MgNB.

In the receiver, the SgNB is provided with a function of routing thedata from each DU to the MgNB. The function of routing the data fromeach DU to the MgNB may be one of the routing functions between DUs.Alternatively, the function of routing the data from each DU to the MgNBmay be one of the functions of transmitting data to the routingdestination DU. The data from each DU is routed to the MgNB in order ofarrival as a function of the receiver. In other words, the data fromeach DU is transmitted one by one to the MgNB in order of arrival.

As an alternative method, the data from each DU may be routed to theMgNB in order of PDCP sequence number (PDCP-SN). In other words, thedata from each DU is routed in order of PDCP-SN and transmitted one byone to the MgNB.

The MgNB may be provided with a function of transmitting, to ahigh-level function, data transmitted from the SgNB. Here, the MgNB mayremove information that is unnecessary for a high-level function, andtransmit the data after removal to the high-level function. The functionof transmitting, to a high-level function, data transmitted from theSgNB may be one of the routing functions between DUs. Alternatively, thefunction of transmitting, to a high-level function, data transmittedfrom the SgNB may be one of the functions of determining a routingdestination DU.

Consequently, the receiver can transmit data from each DU of the SgNB tothe MgNB.

The methods for configuring the routing functions between DUs that aredisclosed in the sixth embodiment should be appropriately applied to themethods (1) to (3) for configuring the function of determining a routingdestination DU and the methods (1) to (3) for configuring the functionof transmitting data to the routing destination DU.

FIG. 48 illustrates an example architecture when the MgNB is providedwith the function of determining a routing destination DU of the SgNBand the PDCP in the CU of the SgNB is provided with the routingfunctions between DUs. FIG. 48 illustrates that both the MgNB and theSgNB have the CU-DU split configurations in Option 2.

The MgNB includes a CU and DUs. The two DUs (the DUs #1 and #2) areconnected to the one CU. The CU includes the RRC, the New AS layer, andthe PDCP. Each of the DUs includes the RLC, the MAC, and the PHY.

The SgNB includes a CU and DUs. The two DUs (the DUs #1 and #2) areconnected to the one CU. The CU of the SgNB includes the RRC. Each ofthe DUs includes the RLC, the MAC, and the PHY. Since the SgNB is asecondary gNB of the UE performing the DC, the CU does not include theNew AS layer and the PDCP in the conventional configuration. In thefirst modification of the sixth embodiment, however, the CU includes thePDCP.

The function of determining a routing destination DU of its own gNB andthe function of transmitting data to the determined routing destinationDU are provided in the PDCP in the CU of the MgNB. The routing functionsbetween DUs may be provided instead of the function of determining arouting destination DU and the function of transmitting data to thedetermined routing destination DU.

The function of determining a routing destination DU of the SgNB isprovided in the PDCP in the CU of the MgNB. The function of transmittingdata to a routing destination DU is provided in the PDCP in the CU ofthe SgNB.

The MgNB notifies the SgNB of information indicating the routingdestination DU determined by the MgNB, in association with the data tobe transmitted to the SgNB. Upon receipt of the information, the SgNBcan recognize which DU the SgNB should transmit the data transmittedfrom the MgNB to.

Consequently, the SgNB can transmit, to the routing destination DUdetermined by the MgNB, the data transmitted from the MgNB.

The interface disclosed in the sixth embodiment should be applied to aninterface for data communication between the MgNB and the SgNB.

In the receiver, data from each DU is routed to the MgNB with thefunction of routing the data from each DU to the MgNB which is afunction provided in the PDCP configured in the CU of the SgNB. Forexample, the SgNB routes and transmits the data from each DU to the MgNBin order of arrival.

The MgNB transmits data from the SgNB to a high-level function with thefunction of transmitting, to the high-level function, data transmittedfrom the SgNB which is a function configured in the MgNB.

Since such a method enables the use of the PDCP that is an existingprotocol stack in the SgNB, the gNB can be easily configured. The gNBwith the CU-DU split configuration in Option 2 is easily used as asecondary gNB in the DC.

Information indicating the presence or absence of the CU-DU splitconfiguration of its own gNB may be notified between the gNBs.

Information on the DUs of its own gNB may be notified between the gNBs.Information on the DUs to be connected to the CU of its own gNB may benotified as the information on the DUs. The information on the DUs maybe notified in association with the information on each DU and the CU tobe connected to the DU.

The methods disclosed in the first modification differ from thosedisclosed in the sixth embodiment by the MgNB having the function ofdetermining a routing destination DU of the SgNB. Thus, a master gNB canrecognize information indicating the presence or absence of the CU-DUsplit configuration of a secondary gNB or information on the DUs. TheMgNB can determine a routing destination DU of the SgNB in considerationof these pieces of information.

The information on the DUs should be DU identifiers.

The DU identifiers may be associated with identifiers of the gNBs or thecells. The information on the DUs may be addresses of the DUs.

For example, a DU identifier includes a gNB identifier and a DUidentifier in the gNB. Alternatively, the DU identifier includes a cellidentifier and a DU identifier in the cell. Consequently, theassociation of the DU identifier with an identifier of a gNB or a cellenables recognizing by which gNB the DU is configured.

The information on the DUs to be notified between the gNBs may belimited to information on the DUs available for transmitting andreceiving data to and from the other gNBs. For example, the informationon the DUs to be notified between the gNBs may be limited to informationon the DUs that can be used in the DC. Consequently, the amount ofinformation to be notified between the gNBs can be reduced.

The information on the CU to be connected to the DU should be a CUidentifier. The same methods on the DU identifier should be appliedthereto.

The information indicating the presence or absence of the CU-DU splitconfiguration or the information on the DUs may be notified between thegNBs upon setting up of an interface between the gNBs. The informationmay be notified to the other gNBs by including, in a set-up message, theinformation indicating the presence or absence of the CU-DU splitconfiguration of its own gNB or the information on the DUs.

Alternatively, the information may be notified upon update of the gNBsetting. The information may be notified to the other gNBs by including,in an update message for the gNB setting, the information indicating thepresence or absence of the CU-DU split configuration of its own gNB orthe information on the DUs.

The other gNBs may be surrounding gNBs.

Such notifications of the information upon setting up the interface orupon update of the gNB setting enable the MgNB to recognize theconfiguration of the DU of a gNB to be used as a secondary gNB uponsetting of the DC. Thus, the MgNB can determine the routing destinationDU of the SgNB.

The information indicating the presence or absence of the CU-DU splitconfiguration of its own gNB and the information on the DUs of its owngNB may be notified upon setting of the DC. The method for notifying,from the SgNB to the MgNB, information indicating the presence orabsence of the CU-DU split configuration of its own SgNB upon setting ofthe DC, which is disclosed in the sixth embodiment, should be appliedthereto.

The MgNB may notify the SgNB of information requesting the use of theDUs of the SgNB for the DC upon setting of the DC. The information mayinclude the number of DUs to be preferably used in the DC.

For example, upon SgNB addition, the MgNB may notify the SgNB of therequest. The request may be included in the signaling for the SgNBaddition to be requested from the MgNB to the SgNB.

Alternatively, upon SgNB modification, the MgNB may notify the SgNB ofthe request. The request may be included in the signaling for the SgNBmodification to be requested from the MgNB to the SgNB.

Consequently, the SgNB can recognize the number of DUs required by theMgNB in the DC. The SgNB can prevent the extra number of DUs from beingset in the DC. The SgNB can also prevent the insufficient number of DUsfrom being set in the DC.

Upon setting of the DC, the SgNB may notify the MgNB of information onthe DUs available for the DC in the SgNB. Examples of the information onthe DUs include identifiers of the DUs and the number of available DUs.

For example, upon SgNB addition, the SgNB may notify the MgNB of theinformation. The information may be included in the signaling fornotifying a response to the request for the SgNB addition.

Alternatively, upon SgNB modification, the SgNB may notify the MgNB ofthe information. The information may be included in the signaling fornotifying a response to the request for the SgNB modification.

Consequently, the SgNB can notify the MgNB of the number of availableDUs in the DC according to the resources in its own gNB or a load state.Since the MgNB can recognize the number of available DUs of the SgNB inthe DC, it can appropriately perform the routing to each gNB for the SB.

The buffer configuration of the SgNB and information to be notifiedbetween the MgNB and the SgNB are disclosed.

One buffer for routing is provided in the CU of the SgNB. The buffer maybe provided inside or outside the PDCP in the CU of the SgNB. The buffershould be provided between the function of determining a routingdestination DU which is configured in the MgNB and the function oftransmitting data to the routing destination DU which is configured inthe CU of the SgNB. The buffer may be set higher than the function oftransmitting data to the routing destination DU which is configured inthe CU of the SgNB.

FIG. 49 illustrates an example architecture when one buffer for routingis provided in the CU of the SgNB. FIG. 49 illustrates the example ofproviding the buffer outside the PDCP in the CU of the SgNB. The bufferis set higher than the function of transmitting data to the routingdestination DU which is configured in the CU of the SgNB.

When the DC with the SB is performed on the target UE, temporary storageof the data transmitted from the MgNB in the buffer provided in the CUof the SgNB can reduce the data loss caused by delay in the followingfunction of transmitting data to the routing destination DU.

The same buffer may be provided in the receiver. Temporary storage ofthe data received from each DU in its transmission to the MgNB canreduce the data loss in the interface for transmitting data from theSgNB to the MgNB.

The information to be notified from the MgNB to the SgNB includesinformation associated with the DL data. The information associated withthe DL data is, for example, a sequence number assigned through theinterface between the MgNB and the SgNB. When Xn or Xx is provided, thesequence number assigned through the interface should be the informationassociated with the DL data. Examples of the sequence number includeXn-U SN and Xx-U SN.

Information indicating a routing destination DU of the SgNB may benotified as the other information to be notified from the MgNB to theSgNB. The information indicating a routing destination DU of the SgNBmay be included in the information associated with the DL data to benotified. The MgNB can route, to the routing destination DU determinedby the MgNB, the data transmitted to the SgNB.

The sequence number assigned through the interface between the MgNB andthe SgNB may be assigned for each routing destination DU. The SgNB candetermine, for each DU, data lost through the interface between the MgNBand the SgNB.

Information indicating a transmission target gNB may be notified as theother information to be notified from the MgNB to the SgNB. Informationindicating a transmission source gNB may be notified as the otherinformation. With application of a plurality of SgNBs, the SgNB canverify whether received data is data transmitted to its own gNB or fromwhich MgNB the received data has been transmitted. Thus, themalfunctions in the DC with the SB with application of a plurality ofSgNBs can be reduced.

When the DC with the SB is performed with application of a plurality ofSgNBs, the routing functions for the SB in the MgNB should determinewhich SgNB the MgNB routes transmission data to.

The information disclosed in the sixth embodiment, specifically, theinformation on a transmission state of the DL data to be notified fromthe SgNB to the MgNB should be applied to the information to be notifiedfrom the SgNB to the MgNB. The same advantages are produced.

In the information to be notified from the SgNB to the MgNB that isdisclosed in the sixth embodiment, the PDCP-SN successfully transmittedor having failed in transmission need not be notified to the UE. Thiscan reduce the information to be notified from the SgNB to the MgNB. TheMgNB determines the PDCP-SN successfully transmitted or having failed intransmission from the SgNB to the UE.

For example, the PDCP of the MgNB assigns the PDCP-SN to data before therouting for the SB, and manages which gNB the data has been routed toafter the routing for the SB. The UE transmits a status report of thePDCP to the MgNB. The UE may transmit the status report through the SgNBor directly to the MgNB. Since this enables the use of the gNB or the DUwith superior communication quality, the reliability of thecommunication is increased.

Consequently, the MgNB can determine the PDCP-SN successfullytransmitted or having failed in transmission from the SgNB to the UE.

Since the SgNB need not determine the PDCP-SN successfully transmittedor having failed in transmission to the UE, the functions of the SgNBcan be reduced, and the SgNB can be easily configured.

In the information to be notified from the SgNB to the MgNB that isdisclosed in the sixth embodiment, the information on a buffer size neednot be notified. This can reduce the information to be notified from theSgNB to the MgNB. For example, the MgNB makes the bearer setting for theSgNB, and measures the amount of PDCP-PDU transmitted to the SgNB.

Consequently, the MgNB can determine, for example, the information on adesired buffer size for a bearer to be the SB in the SgNB and theinformation on the minimum desired buffer size for the UE dependent onthe SB.

Since the SgNB need not determine, for example, the desired buffer sizefor the bearer to be the SB in the SgNB and the minimum desired buffersize for the UE dependent on the SB, the functions of the SgNB can bereduced, and the SgNB can be easily configured.

The information to be notified from the SgNB to the MgNB may be anamount indicating buffer allowance in the SgNB. The SgNB determines thefree capacity of the buffer in the SgNB, and notifies it to the MgNB.Consequently, the MgNB can route the PDCP-PDU for the SB inconsideration of the amount of buffer allowance in the SgNB.

The amount of buffer allowance in the SgNB may be an amount indicatingbuffer allowance for a bearer to be the SB. Alternatively, the amountindicating buffer allowance in the SgNB may be an amount indicatingbuffer allowance for the UE dependent on the SB.

The other examples of the buffer configuration of the SgNB and theinformation to be notified between the MgNB and the SgNB are disclosed.

A buffer for routing is provided in the CU of the SgNB for each DU. Thebuffer may be provided inside or outside the PDCP in the CU of the SgNBor in the RP outside the PDCP in the CU. The buffer should be setbetween each DU and the function of transmitting data to the routingdestination DU which is configured in the CU of the SgNB. The buffer maybe set lower than the function of transmitting data to the routingdestination DU which is configured in the CU of the SgNB.

FIG. 50 illustrates an example architecture when the buffer for routingis provided in the CU of the SgNB for each DU. FIG. 50 illustrates theexample of providing the buffer outside the PDCP in the CU of the SgNB.The buffer is set, for each DU, lower than the function of transmittingdata to routing destination DUs which is configured in the CU of theSgNB.

When the DC with the SB is performed on the target UE, data transmittedfrom the MgNB is distributed and transmitted to the routing destinationDUs determined by the MgNB with the function of transmitting data to therouting destination DUs. Temporary storage of the data transmitted toeach DU in the buffer can reduce the data loss caused by, for example,delay in the interface for transmitting the data to the DU.

The same buffers may be provided in the receiver. Temporarily storing,for each DU, the data received from the DU enables the data receivedfrom the DUs to be easily sorted in order. For example, upon receipt ofdata from two DUs with the same timing, buffering the data enables thereceived data to be sorted in order without any loss. Thus, the SgNB canreduce the data loss in transmission of data to the MgNB.

The information to be notified from the MgNB to the SgNB is, forexample, information identical to the information disclosed on theconfiguration for providing one buffer. This produces the sameadvantages.

The information to be notified from the SgNB to the MgNB is, forexample, information identical to the information disclosed on theconfiguration for providing one buffer. This produces the sameadvantages.

The information to be notified from the SgNB to the MgNB may beinformation for each DU.

The information for each DU may be, for example, information indicatingwhich DU requests data. The RLC in the DU may request data from the PDCPin the MgNB. In such a case, the RLC in the DU should request data foreach DU. The RLC for each DU should request data from the PDCP in theMgNB. Further, the information for each DU may be information on the DUthat has requested data from the RLC in the DU to the PDCP. Theinformation requesting the downlink data from the RLC of each DU whichis disclosed in the fifth embodiment may be applied as an example of theinformation.

The information to be notified from the SgNB to the MgNB may beinformation on a transmission state of the DL data for each DU.

The information to be notified from the SgNB to the MgNB may beinformation on the buffer size for each DU. Examples of the informationinclude information on a buffer size for each DU that is desired for abearer to be the SB and information on the minimum buffer size for eachDU that is desired for the UE dependent on the SB.

The information to be notified from the SgNB to the MgNB may be anamount indicating buffer allowance for each DU. The amount indicatingbuffer allowance for each DU may be an amount indicating bufferallowance for each DU for a bearer to be the SB. Alternatively, theamount indicating buffer allowance for each DU may be an amountindicating buffer allowance for each DU for the UE dependent on the SB.

Defining the information to be notified from the SgNB to the MgNB as theinformation for each DU enables the MgNB to be notified of acommunication state or a buffer state for each DU of the SgNB. Thisenables consideration of the information for each DU in the function ofdetermining routing destination DUs of the MgNB. Thus, the routingdestination DUs can be precisely determined according to a state foreach DU of the SgNB.

FIGS. 51 to 53 illustrate an example sequence on the DC with the SB,using the SgNB with the CU-DU split configuration which is disclosed inthe second modification. FIGS. 51 to 53 are connected across locationsof borders BL5152 and BL5253. Since the sequence illustrated in FIGS. 51to 53 includes the same steps as those of the sequence illustrated inFIGS. 45 to 47, the same step numbers are assigned to the same Steps andthe common description thereof is omitted.

The SgNB has the CU-DU split configuration in Option 2, and two DUs (theDUs #1 and #2) are connected to one CU. The MgNB has also the CU-DUsplit configuration in Option 2, and two DUs (the DUs #1 and #2) areconnected to one CU.

In Step ST6901, the MgNB CU notifies the SgNB CU of a setup requestmessage between gNBs. This message may include the informationindicating the presence or absence of the CU-DU split configuration ofits own gNB and the information on the DUs of its own gNB that arepreviously described.

In Step ST6902, the SgNB CU notifies the MgNB CU of a setup responsemessage between gNBs. This message may include the informationindicating the presence or absence of the CU-DU split configuration ofits own gNB and the information on the DUs of its own gNB that arepreviously described.

In Step ST6808, the MgNB CU notifies the SgNB CU of an SgNB additionrequest message. This message may include the aforementioned informationrequesting the use of the DUs of the SgNB for the DC.

In Step ST6810, the SgNB CU notifies the MgNB CU of an SgNB additionrequest response message. This message may include the aforementionedinformation on the DUs available for the DC in the SgNB.

In Step ST6816, the MgNB CU routes data to be transmitted from thehigh-level NW device, to the MgNB and the SgNB for the SB.

In Step ST6827, the MgNB CU routes, between the DUs, the data routed tothe MgNB. Since this is the same as Step ST6827 in FIG. 47, thedescription thereof is omitted.

In Step ST6903, the MgNB CU determines routing destination DUs of theSgNB for the data routed to the SgNB.

In Step ST6821, the MgNB CU transmits, to the SgNB CU, the data routedto the SgNB. Here, the MgNB CU transmits information on the routingdestination DUs determined in Step ST6903 in association with the data.

Upon receipt of the information on the determined routing destinationDUs and the data, the SgNB CU transmits the data to the routingdestination DUs in Step ST6904.

With execution of the function of transmitting data to the routingdestination DUs in Step ST6904, the SgNB CU transmits the data to the DU#1 in Step ST6823. In Step ST6824, the SgNB DU #1 processes the receiveddata and transmits the processed data to the UE.

With execution of the function of transmitting data to the routingdestination DUs in Step ST6904, the SgNB CU transmits the data to the DU#2 in Step ST6826. In Step ST6825, the SgNB DU #2 processes the receiveddata and transmits the processed data to the UE.

When the UE performs transmission, operations reverse to these areperformed. Thus, the description thereof is omitted.

The aforementioned information to be notified between the MgNB and theSgNB may be notified in the communication between the MgNB CU and theSgNB CU in Step ST6821.

Consequently, the gNB with the CU-DU split configuration in Option 2 canbe used as the secondary gNB to perform the DC with the SB.

Since the methods disclosed in the first modification eliminate the needfor the SgNB CU to determine the routing, the amount of processing inthe SgNB CU can be reduced.

Although the buffer function is provided outside the PDCP in the CU ofthe SgNB, it may be provided inside the PDCP in the CU of the SgNB.Since an existing protocol is provided with a function, the CU of a gNBcan be easily configured.

Alternatively, a new protocol stack having a buffer function may beprovided in the CU of the SgNB. Configuring such a new protocol stackseparately from the existing protocol stacks such as the PDCP enables agNB to be configured without influencing the existing protocol stacks.The malfunctions on the buffer function can be reduced.

The new protocol stack may be, for example, the RP disclosed in thesixth embodiment. The function may be configured in the RP. Integratingthe functions on the routing between DUs into one protocol stack canfacilitate the control and reduce the malfunctions.

The buffer function may be provided in the same portion as the portionprovided with the function of transmitting data to the routingdestination DUs. The processing coordinated with the function oftransmitting data to the routing destination DUs can be easily performedwith low latency.

The second modification of the sixth embodiment

The second modification discloses the other methods for solving theproblems described in the sixth embodiment, specifically, the problemswhen the gNB with the CU-DU split configuration in Option 2 is appliedas the SgNB for the SB.

The MgNB is provided with the routing functions between DUs of the SgNB.The MgNB performs the routing between DUs of the SgNB. The routingfunctions between DUs of the SgNB include a function of determining arouting destination DU of the SgNB, and a function of transmitting datato the routing destination DU.

The routing functions between DUs of the SgNB may be set lower than theconventional PDCP functions or the PDCP functions in the CU which areproposed in NR (Non-Patent Document 22: R3-170266).

When the PDCP has the routing functions for the split bearer, therouting functions between DUs of the SgNB may be set lower than therouting functions for the split bearer. The function of determining arouting destination DU of the SgNB may be provided after the routing tothe SgNB.

Three examples of a method for configuring, in the MgNB, the routingfunctions between DUs of the SgNB are hereinafter disclosed:

(1) the routing functions between DUs of the SgNB are provided insidethe PDCP;

(2) the routing functions between DUs of the SgNB are provided outsidethe PDCP; and

(3) a protocol stack having the routing functions between DUs of theSgNB is provided outside the PDCP.

When the MgNB has the CU-DU split configuration in Option 2, the CU ofthe MgNB is provided with the routing functions between DUs of the SgNB.The CU of the MgNB performs the routing between DUs of the SgNB.

With execution of the function of determining a routing destination DUof the SgNB and the function of transmitting data to the routingdestination DU, the MgNB can transmit, to the routing destination DU ofthe SgNB, the transmission data addressed to the SgNB.

In the receiver, the CU of the MgNB is provided with a function ofrouting data from each DU of the SgNB to a high-level function. Thefunction of routing data from each DU to the high-level function may beone of the routing functions between DUs.

The CU of the MgNB routes the data from each DU of the SgNB to thehigh-level function in order of arrival as a function of the receiver.In other words, the CU of the MgNB transmits the data from each DU ofthe SgNB one by one to the high-level function in order of arrival.

As an alternative method, the CU of the MgNB may route the data fromeach DU of the SgNB to the high-level function in order of PDCP sequencenumber (PDCP-SN). In other words, the CU of the MgNB routes the datafrom each DU of the SgNB in order of PDCP-SN and transmits the data oneby one to the high-level function.

Consequently, the MgNB can transmit the data from each DU of the SgNB tothe high-level function also in the receiver.

The methods for configuring the routing functions between DUs which aredisclosed in the sixth embodiment should be appropriately applied to themethods (1) to (3) for configuring the routing functions between DUs ofthe SgNB.

FIG. 54 illustrates an example architecture when the MgNB is providedwith the routing functions between DUs of the SgNB. FIG. 54 illustratesthat both the MgNB and the SgNB have the CU-DU split configurations inOption 2.

The MgNB includes a CU and DUs. The two DUs (the DUs #1 and #2) areconnected to the one CU. The CU includes the RRC, the New AS layer, andthe PDCP. Each of the DUs includes the RLC, the MAC, and the PHY.

The SgNB includes a CU and DUs. The two DUs (the DUs #1 and #2) areconnected to the one CU. The CU of the SgNB includes the RRC. Each ofthe DUs includes the RLC, the MAC, and the PHY. Since the SgNB is asecondary gNB of the UE performing the DC, the CU does not include theNew AS layer and the PDCP.

The PDCP in the CU of the MgNB is provided with the functions ofdetermining a routing destination DU of its own gNB and the functions oftransmitting data to the determined routing destination DU. The routingfunctions between DUs may be provided instead of the functions ofdetermining a routing destination DU and the functions of transmittingdata to the determined routing destination DU.

The PDCP in the CU of the MgNB is provided with the routing functionsbetween DUs including the functions of determining a routing destinationDU of the SgNB and the functions of transmitting data to the routingdestination DU. The routing functions between DUs of the SgNB are setlower than the routing functions for the split bearer in the PDCP.

With the routing functions for the SB in the PDCP, transmission dataaddressed to the UE performing the DC with the SB is routed to the MgNBand the SgNB. After the routing to the SgNB, a routing destination DU isdetermined with the function of determining the routing destination DUof the SgNB, and the transmission data addressed to the SgNB istransmitted to the routing destination DU of the SgNB with the functionof transmitting data to the routing destination DU of the SgNB.

The MgNB notifies the function of transmitting data to the routingdestination DU of the SgNB of information indicating the routingdestination DU of the SgNB determined by the function of determining therouting destination DU of the SgNB, in association with the data to betransmitted to the SgNB.

In such a manner, the function of transmitting data to the routingdestination DU of the SgNB can control to judge which DU of the SgNB thetransmission data should be transmitted to. The MgNB can route andtransmit data to the DU of the SgNB.

An interface directly connected between the MgNB and the DU of the SgNBshould be provided for data communication between the MgNB and the DU ofthe SgNB. When the MgNB has the CU-DU split configuration in Option 2,an interface directly connected between the CU of the MgNB and the DU ofthe SgNB should be provided. The interface may be a new interface or theinterface between the CU and the DU in the gNB which is disclosed in thefirst embodiment. Alternatively, the interface may be the Fs interfacebetween the CU and the DU in the gNB which is being studied in 3GPP.

In the receiver, the MgNB transmits, to a high-level function, datatransmitted from each DU of the SgNB with the function of routing datafrom each DU of the SgNB to the high-level function. For example, theMgNB routes and transmits the data from each DU of the SgNB to thehigh-level function in order of arrival.

When a split bearer is executed, application of such a method enablesthe SgNB to have the configuration not using the PDCP, similarly as thatwith application of the conventional split bearer. This facilitatesfunctional extension in the SgNB. The gNB with the CU-DU splitconfiguration in Option 2 is easily used as a secondary gNB in the DC.

When a split bearer is executed, transmission of data from the MgNB tothe DU of the SgNB saves transmission of the data to the CU of the SgNB.This can reduce the functions of the CU of the SgNB and facilitate theconfiguration.

A method for setting up an interface between the MgNB and the DU of theSgNB is disclosed.

First, information on the DUs being served by a gNB is notified betweenthe gNBs. The information on the DUs being served by a gNB is, forexample, the information indicating the presence or absence of the CU-DUsplit configuration of its own gNB or the information on the DUs of itsown gNB, which are to be notified between the gNBs and disclosed in thefirst modification of the sixth embodiment. The methods for notifyingthe information that are disclosed in the first modification of thesixth embodiment should be applied to a method for notifying theinformation on the DUs being served by a gNB.

A gNB performs actual setup with all or a part of the DUs notified fromthe other gNBs. The gNB may determine which gNB the setup is performedwith or which DU of the other gNBs the setup is performed with.

The gNB sets up an interface with the DU of a gNB to be connected. Whenthe gNB has the CU-DU split configuration in Option 2, the gNB sets upan interface between the CU of the gNB and the DU of the gNB to beconnected.

The gNB notifies a setup request to the DU of the gNB to be connected.Eleven examples of information to be notified at the setup request arehereinafter disclosed:

(1) an identifier of its own gNB;

(2) an identifier of a serving cell of its own gNB;

(3) an identifier of another cell configured by its own gNB;

(4) an identifier of its own CU of its own gNB;

(5) an identifier of another CU of its own gNB;

(6) an identifier of a DU configured by its own gNB;

(7) an identifier of an adjacent gNB;

(8) an identifier of a cell configured by the adjacent gNB;

(9) an identifier of a CU configured by the adjacent gNB;

(10) an identifier of a DU configured by the adjacent gNB; and

(11) combinations of (1) to (10) above.

Upon receipt of the setup request from the gNB, the DU of the gNBnotifies a setup response to the gNB that has transmitted the setuprequest. Eleven examples of information to be notified in the setupresponse which correspond to the information examples to be notified atthe setup request are hereinafter disclosed:

(1) an identifier of its own gNB;

(2) an identifier of a serving cell of its own gNB;

(3) an identifier of another cell configured by its own gNB;

(4) an identifier of a CU of its own gNB to be connected to its own DUof its own gNB;

(5) an identifier of another CU of its own gNB;

(6) an identifier of another DU configured by its own gNB;

(7) an identifier of an adjacent gNB;

(8) an identifier of a cell configured by the adjacent gNB;

(9) an identifier of a CU configured by the adjacent gNB;

(10) an identifier of a DU configured by the adjacent gNB; and

(11) combinations of (1) to (10) above.

The information to be notified in the setup response may includeinformation indicating a portion that cannot be recognized and a portionthat is lost in the information. This completes the setup between thegNB and the DU of another gNB to be connected.

When the DU of the gNB does not permit a setup upon receipt of the setuprequest, the DU may notify a setup failure to the gNB that hastransmitted the setup request. Examples of the information to benotified in a setup failure include cause information indicating thecause. The information may include information indicating a portion thatcannot be recognized and a portion that is lost in the information.

Notification of the setup failure can prevent the gNB from using the DUof the gNB that has notified the setup failure, for example, in the DC.Consequently, a DU of an accessible gNB can be set as a DU of asecondary gNB for a UE dependent on the DC. This enables leanprocessing.

The gNB may perform actual setup with all or a part of the DUs notifiedfrom the gNB performing the DC. The gNB may determine which DU of thegNB the setup is performed with.

The information and the notification methods that are previouslydescribed should be applied to information to be notified in the setupand the notification method.

Although disclosed is a method for the gNB to set up the DU of anothergNB, the DU of the gNB may set up another gNB. Here, the CU of the gNBmay notify the DU of information indicating a request for setting up theother gNB. Upon receipt of the information from the CU, the DU may starta setup request to the other gNB.

The aforementioned examples of the information to be notified in thesetup response from the DU of the gNB to the other gNBs should beapplied to the information to be notified at the setup request from theDU of the gNB to the other gNBs.

The aforementioned examples of the information to be notified at thesetup request from the gNB to the DU of another gNB should be applied toinformation to be notified in the setup response from the gNB to the DUof the other gNB.

Consequently, the gNB with the CU-DU split configuration in Option 2 canperform the setup.

The gNB should, prior to the DC setting, set up an interface with the DUof a gNB to be connected. Since the setup is already completed uponsetting of the DC, latency for DC setting processing can be reduced.

Upon setting of the DC, the gNB may set up an interface with the DU of agNB to be connected. Here, the interface may be set up after the processof adding or modifying the SgNB. Performing the setup when actuallyrequired can eliminate the wasteful setup processing.

When the DC for another UE is set to a DU that has already been set up,the setup may be omitted. When the DC is set so that a plurality of UEsuse a DU, the setup of the DU may be canceled after all the UEs releasethe DC setting. Appropriate cancellation of the setup with the DUenables release of the resources required for the setup in the DU andthe gNB.

The information indicating the presence or absence of the CU-DU splitconfiguration of its own gNB and the information on the DUs of its owngNB may be notified upon setting of the DC. Upon setting of the DC, theMgNB may notify the SgNB of information requesting the use of the DUs ofthe SgNB for the DC. Upon setting of the DC, the SgNB may notify theMgNB of information on the number of DUs available for the DC in theSgNB. These pieces of information should be notified in the methodsdisclosed in the first modification of the sixth embodiment. Theyproduce the same advantages.

As previously described, the MgNB may set up an interface with the DU ofthe SgNB to be used for the DC, after notification of these pieces ofinformation.

The information to be notified between the MgNB and the SgNB isdisclosed.

The information to be notified from the MgNB to the SgNB includes theinformation associated with the DL data. The information associated withthe DL data is, for example, a sequence number assigned through theinterface between the MgNB and the SgNB DU. When the Fs interface isprovided, a sequence number assigned through the Fs interface should beused. The example sequence number is Fs-U SN.

Information indicating a routing destination DU of the SgNB may benotified as another information to be notified from the MgNB to theSgNB. The information indicating a routing destination DU of the SgNBmay be included in the information associated with the DL data to benotified. The SgNB DU can verify the data addressed to its own DU.

A sequence number may be assigned through the interface between the MgNBand the SgNB DU for each routing destination DU. The SgNB DU candetermine, for each DU, data lost through the interface between the MgNBand the SgNB DU.

The SgNB DU may notify the SgNB CU of the data lost through theinterface.

The SgNB CU may notify the MgNB of information on the data lost throughthe interface between the MgNB and the SgNB DU. When the MgNB has theCU-DU split configuration in Option 2, the SgNB CU may notify the MgNBCU of information on the data lost through the interface between theMgNB CU and the SgNB DU.

The information on the data lost through the interface may be a sequencenumber assigned through the interface.

When a sequence number is assigned through the interface between theMgNB and the SgNB DU for each routing destination DU, the SgNB DU maynotify, for each DU, the SgNB CU of the data lost through the interface.

The SgNB DU may notify, for each DU, the SgNB CU of an identifier of itsown DU in association with the data lost through the interface. Theidentifier is not limited to the identifier of its own DU but may be anyas long as the identifier enables the MgNB to identify which SgNB DU thedata has been lost through the interface with.

The SgNB CU can determine the data lost through the interface for eachDU.

The SgNB CU may notify the MgNB of information on the data lost throughthe interface between the MgNB and the SgNB DU as information for eachDU. The SgNB CU may notify the information in association with anidentifier of each DU. When the MgNB has the CU-DU split configurationin Option 2, the SgNB CU may notify the MgNB CU of information on thedata lost through the interface between the MgNB CU and the SgNB DU asinformation for each DU.

The SgNB CU may collect information for each DU and notify it to theMgNB. The SgNB CU may collect all pieces of information on the DUs to beconnected to the CU and notify them to the MgNB. The SgNB CU may notifythe collected information for each DU, in association with an identifierof the DU. This can reduce the amount of signaling.

The information on the data lost through the interface may be a sequencenumber assigned to each DU through the interface.

Information indicating a transmission target gNB may be notified as theother information to be notified from the MgNB to the SgNB. Informationindicating a transmission source gNB may be notified as the otherinformation. The SgNB DU may notify the SgNB CU of the informationindicating a transmission target gNB and/or the information indicating atransmission source gNB.

With application of a plurality of SgNBs, the SgNB can verify whetherreceived data is data transmitted to its own gNB and which MgNB thereceived data has been transmitted from. Thus, the malfunctions in theDC with the SB with application of a plurality of SgNBs can be reduced.

When the DC with the SB is performed with application of a plurality ofSgNBs, the routing functions for the SB in the MgNB should determinewhich SgNB the MgNB routes transmission data to.

The information to be notified from the SgNB to the MgNB is disclosed.

The information to be notified from the SgNB to the MgNB includesinformation on the data lost in the data communication between the MgNBand the SgNB DU.

The information on lost data is, for example, a sequence number assignedto the lost data through the interface between the MgNB and the SgNB DU.The other examples of the information on lost data include a sequencenumber assigned to data lost first through the interface between theMgNB and the SgNB DU, and a sequence number assigned to data lost lastthrough the interface between the MgNB and the SgNB DU. The SgNB maynotify the number of series of the lost data sets.

When a sequence number is assigned through the interface between theMgNB and the SgNB DU for each routing destination DU, the SgNB maynotify, for each DU, the MgNB of the data lost through the interface.

When the MgNB has the CU-DU split configuration in Option 2, the SgNBnotifies the MgNB CU of these pieces of information.

The SgNB DU may directly transmit these pieces of information to theMgNB. The interface between the MgNB and the SgNB DU should be used.

As an alternative method, the SgNB DU may notify the MgNB of thesepieces of information through the SgNB CU. The transmission methodsdisclosed on the data lost through the interface between the MgNB andthe SgNB DU should be applied to a method for transmitting theinformation to the MgNB through the SgNB CU.

The MgNB should be provided with a buffer for each SgNB DU. When theMgNB has the CU-DU split configuration in Option 2, the CU of the MgNBshould be provided with a buffer for each SgNB DU. The buffer may be setlower than the function of transmitting data to a routing destination DUof the MgNB.

This enables the flow control for each SgNB DU, according to a state ofan interface for each DU between the MgNB and the SgNB DU.

Thus, the MgNB can appropriately and efficiently transmit data to theSgNB DU.

In the second modification, the MgNB transmits data to the DU of theSgNB. The DU has the RLC, MAC, and PHY functions and does not have thePDCP functions. Thus, the SgNB DU cannot determine the successful PDCPSN. The SgNB DU cannot determine a desired buffer size that meetsrequirements of a bearer to be set to the SgNB as the SB.

In the information to be notified from the SgNB to the MgNB which isdisclosed in the sixth embodiment, notification of the PDCP-SNsuccessfully transmitted or having failed in transmission to the UEshould be avoided. The MgNB should determine the PDCP-SN successfullytransmitted or having failed in transmission from the SgNB to the UE.

For example, the PDCP of the MgNB assigns the PDCP-SN to data before therouting for the SB, and manages which gNB the data has been routed toafter the routing for the SB. The UE transmits a status report of thePDCP to the MgNB.

Consequently, the MgNB can determine the PDCP-SN successfullytransmitted or having failed in transmission from the SgNB to the UE.

Since the SgNB need not determine the PDCP-SN successfully transmittedor having failed in transmission to the UE, the functions of the SgNBcan be reduced, and the SgNB can be easily configured.

In the information to be notified from the SgNB to the MgNB which isdisclosed in the sixth embodiment, notification of the information on abuffer size should be avoided. The MgNB makes the bearer setting for theSgNB, and measures the amount of PDCP-PDU transmitted to the SgNB.

Consequently, the MgNB can determine, for example, the information on adesired buffer size for a bearer to be the SB in the SgNB and theinformation on the minimum desired buffer size for the UE dependent onthe SB.

Since the SgNB need not determine, for example, the desired buffer sizefor the bearer to be the SB in the SgNB and the minimum desired buffersize for the UE dependent on the SB, the functions of the SgNB can bereduced, and the SgNB can be easily configured.

The SgNB DU may be provided with a buffer for each DU. The buffer may beset higher than the RLC in the SgNB DU. The buffer should be a bufferfor data transmission between the MgNB and the SgNB DU.

The information to be notified from the SgNB to the MgNB may be anamount indicating buffer allowance in the SgNB DU. The SgNB determinesthe free capacity of the buffer in the SgNB DU, and notifies it to theMgNB. Consequently, the MgNB can route the PDCP-PDU for the SB inconsideration of the amount of buffer allowance in the SgNB DU.

The amount indicating buffer allowance in the SgNB DU may be an amountindicating buffer allowance for a bearer to be the SB. Alternatively,the amount indicating buffer allowance in the SgNB DU may be an amountindicating buffer allowance for the UE dependent on the SB.

When the MgNB has the CU-DU split configuration in Option 2, the SgNBnotifies the MgNB CU of these pieces of information.

The SgNB DU may directly transmit these pieces of information to theMgNB. The interface between the MgNB and the SgNB DU should be used.

As an alternative method, the SgNB DU may notify the MgNB of thesepieces of information through the SgNB CU. The transmission methodsdisclosed on the data lost through the interface between the MgNB andthe SgNB DU should be applied to a method for transmitting theinformation to the MgNB through the SgNB CU.

FIGS. 55 to 57 illustrate an example sequence on the DC with the SBusing the SgNB with the CU-DU split configuration which is disclosed inthe second modification. FIGS. 55 to 57 are connected across locationsof borders BL5556 and BL5657. Since the sequence illustrated in FIGS. 55to 57 includes the same steps as those of the sequence illustrated inFIGS. 51 to 53, the same step numbers are assigned to the same Steps andthe common description thereof is omitted.

The SgNB has the CU-DU split configuration in Option 2, and two DUs (theDUs #1 and #2) are connected to one CU. The MgNB has also the CU-DUsplit configuration in Option 2, and two DUs (the DUs #1 and #2) areconnected to one CU.

In Step ST6901, the MgNB CU notifies the SgNB CU of a setup requestmessage between gNBs. This message may include information on the DUsbeing served by its own gNB.

In Step ST6902, the SgNB CU notifies the MgNB CU of a setup responsemessage between gNBs. This message may include information on the DUsbeing served by its own gNB.

In Step ST6810, the SgNB CU notifies the MgNB CU of an SgNB additionrequest response message with the DC setting. The SgNB CU notifies, viathis message, information on the SgNB DUs available in the DC.

In Step ST7001, the MgNB CU requests the available SgNB DU #1 to set upan interface between the MgNB and the SgNB DU #1.

In Step ST7002, the SgNB DU #1 notifies a setup response to the MgNB CU.

Similarly, in Step ST7003, the MgNB CU requests the available SgNB DU #2to set up an interface between the MgNB and the SgNB DU #2.

In Step ST7004, the SgNB DU #2 notifies a setup response to the MgNB CU.

This completes the setup of the interface provided between the MgNB CUand each of the SgNB DUs. Thus, the communication between the MgNB CUand each of the SgNB DUs is possible.

In Step ST6816, the MgNB CU routes data to be transmitted from thehigh-level NW device, to the MgNB and the SgNB for the SB.

In Step ST6827, the MgNB CU routes, between the DUs, the data routed tothe MgNB. Since this is the same as Step ST6827 in FIG. 52, thedescription thereof is omitted.

In Step ST6903, the MgNB CU determines a routing destination DU of theSgNB for the data routed to the SgNB.

Further in Step ST7005, the MgNB CU transmits the data to the routingdestination DU of the SgNB.

With execution of the function of transmitting data to the routingdestination DUs of the SgNB in Step ST7005, the MgNB CU transmits thedata to the SgNB DU #1 in Step ST7006. In Step ST7007, the SgNB DU #1processes the received data and transmits the processed data to the UE.

With execution of the function of transmitting data to the routingdestination DUs of the SgNB in Step ST7005, the MgNB CU transmits thedata to the SgNB DU #2 in Step ST7008. In Step ST7009, the SgNB DU #2processes the received data and transmits the processed data to the UE.

When the UE performs transmission, operations reverse to these areperformed. Thus, the description thereof is omitted.

The aforementioned information to be notified between the MgNB and theSgNB may be notified in the communication between the MgNB CU and theSgNB CU in Steps ST7006 and ST7008. Although not described herein, theaforementioned information to be notified between the MgNB and the SgNBmay be notified in the communication between the SgNB DU and the MgNB CUthrough the SgNB CU.

Consequently, the gNB with the CU-DU split configuration in Option 2 canbe used as the secondary gNB to perform the DC with the SB.

The methods disclosed in the second modification enable transmission andreception of data directly between the MgNB and the SgNB DU. The CU ofthe SgNB need not have the routing functions between DUs. Thus, theamount of processing in the SgNB CU can be reduced.

The Third Modification of the Sixth Embodiment

In the first and second modifications of the sixth embodiment, the MgNBdetermines the DU of the SgNB to which data transmission is routed. Thissometimes causes a contention between resource use for the UE to whichthe DC with the SB is set and resource use for the other UEs. The SgNBhas difficulties in adjusting the resource use for the UE to which theDC with the SB is set, and the resource use for the other UEs.

Thus, the SgNB has problems of failing to obtain a desired data rate forthe UE to which the DC is set and a desired data rate for the other UEs.

The third modification discloses a method for solving such problems.

The MgNB dedicates all or a part of the DUs of the SgNB. The MgNB maydedicate all or a part of the resources of each DU. The MgNB of one UEor a plurality of UEs to which the DC is set using the same MgNB and thesame SgNB may dedicate the resources for the same DU or for each DU.

A method for dedicating the DU of the SgNB or the resources for the DUto the MgNB is disclosed. The SgNB notifies the MgNB of information onthe DU of the SgNB or the resources for the DU to be dedicated to theMgNB. Alternatively, the SgNB notifies the MgNB of information on the DUof the SgNB or the resources for the DU that the MgNB is permitted todedicate.

Four examples of the information to be notified are disclosed below:

(1) a DU identifier;

(2) resources in the DU;

(3) dedicated time; and

(4) combinations of (1) to (3) above.

On the DU identifier in (1), the SgNB may notify the MgNB of theidentifier of the DU disclosed in the sixth embodiment.

The resources in the DU in (2) include radio resources on the frequencyaxis, radio resources on the time axis, codes, and sequences. These maybe combined. The radio resources on the frequency axis may be used persub-carrier. The radio resources on the time axis may be used per OFDMsymbol. Alternatively, the resources in the DU may be used per slot orper subframe. Examples of the combinations of the resources on thefrequency axis and the time axis include resource blocks.

Orthogonal codes or quasi-orthogonal codes may be used as the codes.Examples of the resources on the codes and the sequences include thecode numbers and the sequence numbers. The code numbers or the sequencenumbers may be any as long as the codes or the sequences are derivable.

Examples of the dedicated time in (3) include information on the timeduring which the DU of the SgNB or the resources for the DU arededicated to the MgNB. Information on, for example, the start time andthe stop time should be used. The unit of time may be a unitrepresenting the resources on the time axis.

For example, a system frame number, a radio frame number, or a subframenumber is specified as the start time. Similarly, a system frame number,a radio frame number, or a subframe number is specified as the stoptime. The MgNB dedicates the DU of the SgNB or the resources for the DUfrom the start time to the stop time.

The dedicated time may be a duration. The duration should be indicatedby, for example, the number of system frames, the number of radioframes, or the number of subframes. For example, a start time and aduration are set, and the setting information is notified from the SgNBto the MgNB. The MgNB dedicates the DU of the SgNB or the resources forthe DU for the duration from the start time.

This enables flexible time settings. The resources of the SgNB can bededicated to the MgNB according to a load state in the SgNB.

A method for notifying the information from the SgNB to the MgNB isdisclosed.

The information may be notified between gNBs upon setting up of aninterface between the gNBs. The interface between the gNBs is notlimited to that between the gNBs, but may be an interface between gNBCUs or between the gNB and the gNB CU. The information on the DU of itsown gNB or the resources for the DU that the other gNBs can dedicate isincluded in a set-up message to be notified.

Alternatively, the information may be notified upon update of the gNBsetting. The information on the DU of its own gNB or the resources forthe DU that the other gNBs can dedicate may be included in an updatemessage for the gNB setting to be notified.

The notification upon setting up the interface or upon update of the gNBsetting is not a notification from the SgNB to the MgNB but anotification from the gNB to the other gNBs. In other words, theinformation is notified before the DC is set. In such a case, the gNBcan set the DC based on the information notified from the other gNBs,and determine a routing destination DU of the transmission data for theSgNB DU.

Another method for notifying the information from the SgNB to the MgNBis disclosed.

The information may be notified upon setting of the DC. The method fornotifying, from the SgNB to the MgNB, information indicating thepresence or absence of the CU-DU split configuration of its own SgNBupon setting of the DC, which is disclosed in the sixth embodiment,should be applied. The information on the DU of its own SgNB or theresources for the DU to be dedicated to the MgNB or that can bededicated to the MgNB should be notified instead of the informationindicating the presence or absence of the CU-DU split configuration ofits own SgNB.

The MgNB can recognize the information on the DU of its own SgNB or theresources for the DU to be dedicated to the MgNB or that can bededicated to the MgNB, through notification from the SgNB to the MgNB.The MgNB may perform the routing for the SB in consideration of these.

The MgNB may notify the SgNB of information requesting dedicated use ofthe DU of its own SgNB or the resources for the DU for the DC uponsetting of the DC. Eight examples of the requesting information aredisclosed below:

(1) an identifier of the UE dependent on the DC;

(2) an identifier of its own gNB;

(3) the number of DUs to be preferably dedicated in the DC;

(4) the amount or number of resources of the DU to be preferablydedicated in the DC;

(5) the time or a duration to be preferably dedicated in the DC;

(6) the carrier frequency to be preferably dedicated in the DC;

(7) the frequency bandwidth to be preferably dedicated in the DC; and

(8) combinations of (1) to (7) above.

The methods for notifying, from the MgNB to the SgNB, the informationrequesting the use of the DUs of the SgNB for the DC upon setting of theDC which is disclosed in the first modification of the sixth embodimentshould be applied to a method for notifying, from the MgNB to the SgNB,information requesting the dedicated use of the DU of the SgNB or theresources for the DU upon setting of the DC. The information requestingthe dedicated use of the DU of the SgNB or the resources for the DUshould be notified instead of the information requesting the use of theDUs of the SgNB.

Consequently, the SgNB can recognize information on the DU or theresources for the DU that the MgNB needs to dedicate in the DC. The SgNBcan prevent the MgNB from dedicating the extra number of DUs in the DC.The SgNB can also prevent the MgNB from dedicating the insufficientnumber of DUs in the DC. The SgNB can dedicate appropriate resources tothe MgNB, and communicate with the UE using the appropriate resources.

The methods disclosed in the third modification enable the DU of theSgNB or the resources for the DU to be dedicated to the MgNB in the DC.Consequently, the DU of the SgNB or the resources for the DU can be usedaccording to a bearer set to the SgNB in the DC. Thus, it is possible toreduce, in the SgNB, the problems of failing to obtain a desired datarate for the set UE and a desired data rate for the other UEs.

The sixth embodiment to the third modification of the sixth embodimentdisclose application of a base station in NR (gNB) as a master basestation in the DC. The methods disclosed in the sixth embodiment to thethird modification of the sixth embodiment may be applied to cases wherea base station in the LTE (eNB) is used as a master base station in theDC.

The eNB is not provided with the New AS layer set higher than the PDCPin the gNB. Further, the CU-DU split configuration is not available inthe eNB.

Thus, the New AS layer of the gNB should be eliminated, and the eNBshould be substituted for the gNB having no CU-DU split configuration.

This produces the same advantages as those described in the sixthembodiment to the third modification of the sixth embodiment.

The DC with the SB can be implemented using the eNB as a master basestation and using the gNB with the CU-DU split configuration in Option 2as a secondary base station. Since the DC can be performed usingexisting base stations in the LTE, the system is easily built at lowcost.

The sixth embodiment to the third modification of the sixth embodimentdisclose the implementation of the DC with the MCG split bearer. Themethods disclosed in the sixth embodiment to the third modification ofthe sixth embodiment may be applied to the DC with the SCG split bearer.

With the SCG split bearer, a bearer is split from a secondary basestation into a master base station and the secondary base station. Datatransmitted from a high-level NW device enters the secondary basestation, and is routed from the secondary base station to the masterbase station and the secondary base station.

Thus, when the master base station is the gNB with the CU-DU splitconfiguration in Option 2, the same problems as those disclosed in thesixth embodiment occur. To solve such problems, the methods disclosed inthe sixth embodiment to the third modification of the sixth embodimentshould be applied to the DC with the SCG split bearer.

The methods disclosed on the master gNB should be applied to a secondarygNB, and the methods disclosed on the secondary gNB should be applied toa master gNB.

This produces the same advantages as those described in the sixthembodiment to the third modification of the sixth embodiment.

The DC with the SCG split bearer can be implemented using the gNB withthe CU-DU split configuration in Option 2 as a master base station. Aschoices of the DC in NR increase, operations of the DC appropriate forthe radio propagation environment, and the high-speed communicationbetween a base station and the UE are possible.

The methods disclosed in the sixth embodiment to the third modificationof the sixth embodiment may be applied to the multi-connectivity (MC).The gNB with the CU-DU split configuration in Option 2 can be an SgNBfor the MC with the SB. Further, the gNB with the CU-DU splitconfiguration in Option 2 may be an MgNB for the MC with the SB. Thesame advantages as those in the DC are produced.

The embodiments and the modifications are merely illustrations of thepresent invention, and can be freely combined within the scope of thepresent invention. Any constituent elements of the embodiments and themodifications can be appropriately modified or omitted.

For example, the subframe in the embodiments and the modifications is anexample time unit of communication in the fifth generation base stationcommunication system. The subframe may be set per scheduling. Theprocesses described in the embodiments and the modifications as beingperformed per subframe may be performed per TTI, per slot, per sub-slot,or per mini-slot.

While the invention is described in detail, the foregoing description isin all aspects illustrative and does not restrict the present invention.Therefore, numerous modifications and variations that have not yet beenexemplified are devised without departing from the scope of the presentinvention.

DESCRIPTION OF REFERENCES

200 communication system, 202, 804 communication terminal device, 203base station device, 801 central unit (CU), 802, 803 distributed unit(DU).

1. A communication system comprising: a communication terminal device,and a base station device configured to perform radio communication withthe communication terminal device, wherein the base station deviceincludes: a plurality of distributed units (DUs) that transmit andreceive radio signals; and a central unit (CU) that controls theplurality of DUs, the communication terminal device transmits, to eachof two or more of the plurality of DUs, a duplicated uplink packet of anuplink packet to be transmitted from the communication terminal device,and upon redundant receipt of the uplink packets, the base stationdevice removes a redundant uplink packet in accordance with a predefineduplink packet removal criterion.
 2. The communication system accordingto claim 1, wherein the CU duplicates a downlink packet addressed to thecommunication terminal device, and forwards the duplicated downlinkpacket to each of at least two DUs among the plurality of DUs, each ofthe at least two DUs transmits, to the communication terminal device bythe radio signal, the downlink packet obtained from the CU, and uponredundant receipt of the downlink packets, the communication terminaldevice removes a redundant downlink packet in accordance with apredefined downlink packet removal criterion.
 3. The communicationsystem according to claim 2, wherein the CU forwards the duplicateddownlink packet to each of at least two DUs determined by the CU amongthe plurality of DUs.